AO3-6212 AO3-6573 Add indices to tag_set_associations, and add the departure gem to make future migrations easier.

[tickinginstant]

Issue

https://otwarchive.atlassian.net/browse/AO3-6212
https://otwarchive.atlassian.net/browse/AO3-6573

Purpose

• Adds the departure gem, so that we can simplify the code for migrations on large tables.
• Moves the PTOSC arguments into the config file, so that it's easier to customize them for different systems.
• Adds three indices to the tag_set_associations table:
  • One on the tag ID.
  • One on the parent tag ID.
  • One on the owned tag set ID, the parent tag ID, and the tag ID. This one is unique.

A couple notes:

• I removed the --no-drop-old-table flag, since that may not be necessary now that the databases are on MariaDB. Because the flags are now controlled through the config files (and can be overridden by specifying the PERCONA_ARGS environment variable), it shouldn't require a PR to add back the flag if MariaDB turns out to have similar problems to MySQL.
• I think this is a good migration to test things with, including departure. It's on a relatively large table, so it probably should use PTOSC, but if something goes catastrophically wrong, it should only affect a relatively small number of users.

Testing Instructions

As on Jira, but since this introduces a new tool, there'll need to be some systems changes prior to the deploys on staging and production.

[sarken]

(james indicated he wanted to look at this when he had time, so I'm tagging him in as an assignee.)