Add language picker to post form

What does this PR do? Continuation of !10602 (merged) and !10603 (merged), fixes #9692. This adds a language picker to the compose form UI that allows users to specify the language a post is in. The "Auto detect" option uses tinyld to detect the post language, much like the current behavior.

This is currently a draft because while this works and can be used, there's still a few more things that can be done to improve it. Here's a checklist of what I can think of at the moment:

• Add search to the language picker dropdown
• Reduce height of dropdown
• Audit the revised langmap
• Unify/use langmap in more places

More testing is also needed to make sure this doesn't break things.

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