Add pin events to user timeline (backend)

[jdaok]

(this PR doesn't depend on any currently opened PR's and can be merged)
This PR contains the backend changes to add pin events to the user timeline.

Changes:

• adds get_pins_for_feed() to pinned_recording.py to retrieve pins for list of users
• adds new class APIPinEvent to APIEventMetadata
• adds get_recording_pin_events() to user_timeline_event_api.py to fetch pins and convert to APITimelineEvents

frontend changes -> #1560

[pep8speaks]

Hello @jdaok! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-08-08 05:00:14 UTC

[mayhem]

This is a bit of a problem here -- given that the fetch_track_metadata_for_pin is in a loop, it means that we're going to do 1 DB query for each pin event, which is far from ideal. We need to support doing this in one query -- one possibility is to refactor the fetch_track_metadata_for_pin to accept a list of pins, rather than just one.

[jdaok]

Thanks, I opened #1570 to refactor the functions and fix this, after that gets approved I'll merge changes here and we should be able to merge this and #1560 into master.

[mayhem]

OK, 1570 is merged.

[mayhem]

Fix the merge conflict on 1560 and lets get this merged!

[mayhem]

Looks very good overall, we just need to refactor that one (two?) function(s).