fixes #3331

Notifications from strangers are disabled by default in order to cope
with spam. On the other hand, this complicates contacting other users
for the first time, which leads to a bad user experience.

Merge branch 'implement_room_destruction' of https://github.com/area-42/Conversations into bookmarks

added epub and azw mime types. try to resolve application/octet-stream by file extension instead. added preview icons for ebooks

* This is sadly needed

* Enable instead of activate