Migrate workshop data on past/current memberships to fit new workshop pricing model

[maneesha]

As we implement #2387 we will need to migrate existing data. This migration should be applied as follows. Allowed workshops should match completed workshops.

Generally speaking, any workshop before March 1 2020 (approximate date of COVID global shutdown) happened in-person so we should treat the "allowed" as in-person.

[maneesha]

Example:
Member X was allowed 4 workshops in their membership, term dates August 1 2019 to July 31 2020. They completed 2 inperson workshops in 2019, and 2 online workshops in 2020. The agreement should read as if they were allowed 2 inperson and 2 online for that agreement term.

For unused benefits : If the membership ended before Feb 29 2020, consider the allowed workshops to be in-person workshops. For all memberships after that, consider the allowed workshops to be online workshops.

We will run reports and decide if we want to make manual adjustments from there.

[maneesha]

Also include auto-generated comment that describes the number of workshops migrated.

[elichad]

This migration will need to be written manually, making use of Django's RunPython migration step. See migration 0261_migrate_lesson_contribution_to_get_involved for a recent example.

1. Write the forward step(s)
2. Write the reverse step(s) - this migration is reversible, so the reverse steps should undo everything that happened in the forward steps
3. Write tests for both forward and reverse migrations with a variety of pre-existing data. Think about as many edge cases as possible. (follow the format in workshops/tests/test_migrations.py)
4. Run the migration forward and backward a couple of times on your dev database to ensure it works properly on slightly more realistic data