Add new [E]dit command to the timeline: opens an existing toot to allow
editing it. Since this is more or less the same operation as posting a
new toot, extend the StatusComposer view to support this rather than
implementing a new view.
Add a new api method, fetch_status_source(), to implement the
/api/v1/statuses/{id}/source endpoint used to fetch the original post
text.
ec48e8eed8