First rough attempt at a global latest changes overview.

In response to https://github.com/gollum/gollum/issues/707. @sunny @dometto @schmunk42 @maxchill This gives a very basic overview of the latest changes in the wiki / git repo (based on the history view). There's no view (yet) to inspect the commits, but the table does give an impression of activity. Please see this as a first proposal to be worked (but not necessarily by me). I would welcome PRs that improve on this code.