Merge branch 'rs-event-titles' into 'master'

Add an Event's target's title to its reference link

Given an activity feed entry like:

> Douwe Maan commented on [issue #123] at [gitlab-org/gitlab-ce]

...the `issue #123` link will now have a `title` attribute.

Plus some minor refactorings, see individual commits for details.

See merge request !4090