Per-wiki templates

[supertassu]

This is built on top of #373. Changes only in this PR.

This will link templates to a certain wiki. It is done in three separate migrations:

• First, add a nullable wiki_id column to the templates table. It has a foreign key on wikis.id with on delete cascade, meaning that the templates will be removed if the wiki is removed. Add an index on wiki_id, active for efficient queries.
• Second, backfill wiki_id to always point to enwiki. This step requires that utrs-maintenance:sync-wikis-to-database has been run if there are any templates in the database. (should be done by a migration)
• Last, remove make wiki_id not nullable as all templates should have a value by now

[dqwiki]

I will also note that there is no way to modify which appeal is assigned to which wiki.