This is still a bit flaky around when to remove a mention, but it should
work in most cases. Might add a toggle in the future to disable these
though.

Fix various downscaled icons by removing undeeded multiplications by devicePixelRatio in sourceSize.

Fix downscaled PL indicator in the timeline by using the actual size as sourceSize.

Fix various blurry icons by specifying sourceSize.

Fixes #1125

More consistent language with similar description strings
Add manpage entry
Add /unignore
Add ignore & decline button in the invite page
Add invite rejection to all ignore methods

Before we only did that, when we basically didn't have the key yet. But
since we usually get sent a RoomKey when a new message is sent after we
sign in, we were discarding, that those messages should usually now be
trusted.