Delete management commands we no longer need

[jacobdgm]

This PR deletes a number of commands in management/commands that are no longer needed. In doing so, it fixes #1168.

It renames and slightly reworks one command (populate_chant_fields has become touch_all_chants), deletes many commands, and retains just a few. For justification for deleting each of the deleted commands, see #1179 (comment) below. For justification for retaining each of the retained commands, see #1179 (comment) below.

[jacobdgm]

Commands deleted:

• update_differentia_new
  • we created a field, differentia_new (now differentiae_database), which hadn't been there when our chant model was initially set up. This command looked at chants' json_info field and took the relevant value from there to populate the new field.
  • this command only needed to be run once, and it has. It's done its job!
• sync_sources
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• sync_source_creation_date
  • we previously hadn't kept track of the time of creation of the sources in CantusDB. This script got this information from one of the APIs on OldCantus and added it to the existing sources on NewCantus. It's been run, and its job is complete.
• sync_sequences
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• sync_indexers
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• sync_genres
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• sync_feasts
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• sync_chants
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• replace_indexer_fields
  • this was a necessary step to merge indexers and users. This merger is complete; this command has done its job
• populate_volpiano_new_fields
  • there are several volpiano-related fields (volpiano_notes, volpiano_intervals) that are used on the Melody Search page. These fields are now kept up-to-date by update_volpiano_fields in signals.py
• populate_next_chant_fields
  • the next_chant field is used to optimize the calculation of feasts that follow a given feast. its value is kept up-to-date by signals.py, and this script no longer needs to be run.
• populate_is_last_chant_in_feast
  • the is_last_chant_in_feast field is used to optimize the calculation of feasts that follow a given feast. its value is kept up-to-date by signals.py, and this script no longer needs to be run.
• populate_chant_fields
  • this PR renames this command from populate_chant_fields to touch_all_chants. The command goes through every chant and saves it, triggering on_chant_save in signals.py. It was meant to update an existing copy of the database while adding new behavior to signals.py. Since this behavior may still be useful in the future as we make further changes to signals.py, it has been retained, and renamed to reflect its purpose.
• populate_chant_and_melody_counts
  • this populates the number of chants and number of chants with melodies for each source in the database. These values are now kept up to date in signals.py, and this command should never need to be run again.
• merge_indexer_user
  • The indexer-user merger is now complete. This command has done its job!
• add_unknown_source
  • it used to be that some chants/sequences had no source, but Source is now a required field for those models. This command created two "Unknown Source" sources, one for chants and one for sequences, and assigned sourceless chants/sequences to these sources. This command has already run to completion on Production, and running it would both cause problems (creating duplicate "Unknown Source" sources) and pointless (since all chants/sequences already have sources)
• add_prefix
  • this command added feast codes and prefixes to each Feast in the database. The feast code functionality was added to the sync_feasts command, and the prefix field is kept up-to-date by a signal in signals.py. This command is now several levels of out-of-date.
• add_all_users
  • one of the scripts for syncing data from OldCantus to New. It's done its job!
• add_differentiae
  • This just ran to completion on Production, and seems to have worked. Its job is done!

[jacobdgm]

Commands that I left:

• add_cantus_ultimus_links
  • this is implicated in On Chant Detail page, we should add links to corresponding Cantus Ultimus page #429, We need a workflow for attaching image links to records #940, Source Detail Page/Sidebar: Allow for multiple image views (Cantus Ultimus and host institution) #1079
• remap_user_ids
  • this is currently in the workshop, as it's needed to fix remap_user_ids command is not working properly #1165
• touch_all_chants
  • this command was renamed from populate_chant_fields. See Delete management commands we no longer need #1179 (comment) for further details.

[dchiller]

Approving...everything makes sense.