Support editing via client API

[andrewgodwin]

This may need us to properly solve the multipart-form-data-on-not-POST problem first though.

[chdorner]

Just did a basic test for this with Ivory and it generally seems to work. It turned a local post back into a public post, but fairly sure that's because Ivory doesn't understand local posts.

Is there anything that's still left to do for editing?

[andrewgodwin]

I don't think so - this just didn't get closed properly. The local/public thing will happen because we need to squash post visibility values down to the Mastodon set, yeah.