No uniqueness control on AspectVisibility resulted in possible having
multiple AspectVisibility objects in the DB for the same aspect and
shareable which doesn't make sense. Introduce uniqueness validation
and fix up tests where duplication happened.
markdown link to their profile (fixes#2516)
add failing spec for #4160 / #2516
extend the spec a bit more
refactor mention handling in a status message
add method for filtering mentions by aspects
wire mention filtering into the status message model, adapt a few tests to
work properly
cosmetic changes
shorten helper methods
add changelog entry
