AO3-6125 Make admin-post sorting by language case-insensitive

[brianjaustin]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6125

Purpose

1. Adds a before task (new concept, things to be run before a deploy) to backfill languages with a blank sortable name
2. Changes the sortable name for languages to be required
3. Updates the way translations for admin posts are fetched to be sorted by their language's sortable_name

Credit

Brian Austin (they/he)

[redsummernight]

Should we fall back to name instead of short?

[brianjaustin]

Maybe, although this could require a fair amount more in terms of test changes (using short was how the behaviour used to work, so at least a few tests rely on that)

[redsummernight]

Yeah, a TODO should be fine for now.