Merge branch 'bugfix/star_cache' into 'master'

Touch project when toggling stars to update cache

This is a fix for gitlab-org/gitlab-ce#3513.

When a star toggle event happens, we touch project to clear cache key.
This will trigger a new "updated_at" at the project model. If it's undesirable, please let me know and I will try to solve it in a different way.

See merge request !1970