Add <mentions:someuser> filter

[nikku]

Is your feature request related to a problem? Please describe

GitHub has a mentions filter that allows me to get an overview over all attempts to pull me into a topic.

The filter is handy in certain situations, i.e. when I'm working with colleagues in a collaborative, asynchronous fashion.

Describe the solution you'd like

• mentions:username is available in search
• involves:username includes mentions

Describe alternatives you've considered

None.

Additional context

Depends on #94.