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

This further reduces the minimum API level to 16, which should encompass
most users stuck on older versions of Android (mainly BlackBerry OS and
Jolla users).

Several issues reported by code analysis were fixed, mainly around issues
with layouts.

Message styling purposly doesn’t require a whitespace after a closing
tag to make something like ~un~believable work. However it also breaks
_Programmierer_innen_ and other example where the tag is repeated as a
non tag in the word.
Therefor we change the rules that if a closing tag is followed by a higher
order closing tag (a closing tag followed by an end block or white space)
we ignore the first closing tag. But only if we don’t read another tag open.