StatusComposer usability fixes

This is a mega-fix collection of a bunch of status composer papercuts I encountered :-)

Only drop the Status Composer if a toot is sent

Right now the status composer is closed immediately, without any feedback as to whether or not the toot was sent. The user is thrown back to the timeline, and they don't see their toot - thinking that it never sent. Now, the composer closes only once we get the reply back from Mastodon - and usually at that point the timeline has updated with their post.

Fix media upload indicator

The network progress object has the same double NOTIFY issue, meaning that the upload progress bar never went away. Now that's fixed, and the signals are simplified. Also, there is now an "indeterminate" stage where the upload progress is technically 100%, but Mastodon isn't done processing the image. That's now visible, and it doesn't look like something went wrong:

Display an error when sending a toot

This should never happen, but in the rare case where Mastodon encounters an error (and hopefully gives us a message) it's now displayed in the composer:

Don't allow posting a toot before media is uploaded

When you have media still in the process of being uploaded, it now disables the button so you don't accidentally send the toot prematurely.