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

.eslintignore.errorfiles

@@ -52,7 +52,6 @@ src/components/views/settings/ChangePassword.js
 src/components/views/settings/DevicesPanel.js
 src/components/views/settings/IntegrationsManager.js
 src/components/views/settings/Notifications.js
-src/ContentMessages.js
 src/GroupAddressPicker.js
 src/HtmlUtils.js
 src/ImageUtils.js