FIX: now, only most recent PageVisit is updated

- When activity updates are received, the url is used to search the
  NewPageVisits collection for an appropriate PageVisit. PageVisits
  are sorted by descending by creation time, and the first one in that
  result set gets its updated_at attribute updated.
- However, there is no check for a maximum allowable amount of time on
  a particular tab. As long as the content script in that tab is
  sending activity updates, the most recent tab with that tab's url is
  updated.
Merge branch 'develop' of github.com:Slimformation/slimformation into develop