Add post `content_type` selector to the compose box

[Steffo99]

Currently, specific to https://github.com/cheeaun/phanpy/labels/akkoma .

Waiting for more info on the current implementation status of the feature on https://github.com/cheeaun/phanpy/labels/pleroma .

Likely that other server implementations have this feature as well; will add those as soon as I get an account on each of them, but will probably come in a different pull request.

[Steffo99]

Could use a "Default content type" option in the settings menu, as one already exists in Akkoma.

[Steffo99]

Seems like a list of supported post content_types is returned in /instance at .pleroma.metadata.post_formats. Maybe it might be worth populating the list from there?

[tsmethurst]

GoToSocial supports this flag too, just so you know :) For text/plain and text/markdown at least.

[Steffo99]

This should now work with any Mastodon API implementation!

I've tried it only on Mastodon Glitch Edition / Chuckya Edition, since that's the only other software I have accounts on at the moment.

I guess I could use accounts on more instances!

[Steffo99]

This needs a bit more testing I think, reverting to draft.