Ability to schedule-post a saved draft (or to unschedule a post by converting it to a draft)

[nekohayo]

Describe the request

Currently, with Tuba 0.10.3, when you create a new post you have the option to either post it, schedule it, or save it as a draft.

However, if you save it as a draft, later when you go into the drafts list you can only edit or post it "right away" (and those two modes are not very easily discovered, as seen in #1623); you cannot decide to convert a drafted post into a scheduled post (or vice versa). It would be nice if that was possible, even if that means inheriting the draft into a scheduling window and once successfully scheduled (and not cancelled) then it clears the old draft behind the scenes...

Checks

• This follows the GNOME HIG.

[GeopJr]

Not opposed, not a lot of thought went into the UX of everything that was added to the composer after the new composer mockups landed (couldn't keep delaying it anymore :P)

On the technical side:

• Both scheduled and draft posts are actually scheduled posts. There's no API for drafts, so what clients do is schedule them for 3000 years in the future (sorry if anyone's drafts get posted in 3000 years)
• The are not done locally because of attachments and such (the serve keeps them saved and synced between devices and installations)
• I can't remember from the top of my head but some stuff around editing scheduled posts are iffy. There's no API for editing them so we just create a new scheduled post and delete the old one, but there's some race conditions on the server which forces is to do it like this. If you remove the old scheduled post first, the attachments (on the server) can be removed and we won't be able to re-attach them to the new post without re-uploading them (and we can't do that because we might not have them as files here (scheduled post from another device, from a different session etc etc))