replace mentions of out-of-aspect people in limited posts with just a
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
Merge pull request #4161 from Raven24/issue_4160

evil mentions