The banners shown in place of the composer for invited and left rooms
suffered from numerous problems due to being written very long ago and
untouched since then: unflexible components, layout glitching at certain
size, and focus issues.
These elements have been reimplemented as part of the Composer using
standard components, handle lack of space/width much better and
correctly take focus when switching rooms or the room's state changes.
Other note, the inviter's avatar is no longer shown in place of where
the current writing user is currently shown, to maintain consistency and
keep an indication of which account the user is acting as.
The inviter's profile should be available in the right pane, but nio
apparently doesn't give us member events for invited rooms.
In the composer, when the text cursor was inside a word starting
with a @ but not matching any username (thus the completion UI being
invisible), pressing up/down arrow would not do anything instead of
going to the prev/next line.
Simplify the code, make it more declarative, and improve the reliability
of sending typing notifications (notably when changing the used alias in
the middle of a message).
thumbnail() and download() were remaining.
Use a direct and carefully chosen MatrixClient's methods instead to
avoid problems mentioned in the previous commit 7502c1.
