There was a minor copy-paste issue with publisher_public and
publisher_explain methods. Fix it and do a little refactoring
of the publisher's code.

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

publisher