Throttle updates to `projects.last_activity_at`

The column projects.last_activity_at is updated every time an event is created. This means that for every comment, every issue, every push, every merge request, basically every thing that's created we update this column. Per https://gitlab.com/gitlab-com/infrastructure/issues/6#note_15373026 this results in around 7773 updates in just an hour. Because this column is used (e.g. for sorting a projects list) we can't outright remove it (unless we're OK with that). As such I propose that we only update this column for a project if it wasn't already updated in a certain time frame. A short interval such as 5 minutes can already reduce the number of writes drastically. We can take this a step further and only update once every hour, but this may be too conservative.

@JobV @DouweM Since this may sorta impact our feature set (I'm not sure how crucial this timestamp is), are there any objections against doing this?