Implement redesigned upload confirmation screens
Also fairly significant refactor of the uploading code: there are
a number of different ways of triggerring a file upload and each
went through a different code path (the media config size limit
worked on one of those paths). Basically take a lot of code out
of the views and put it into ContentMessages.
Sorry about the size of this patch.
https://github.com/vector-im/riot-web/issues/7565
