Better source and commits navigation (BB-3555)

This issue is about more tight connection of source and commits browsing. Currently the file links at the commit details screen lead to file history, but there is no possibility to show a file at a certain revision using only bitbucket's web interface except for manually typing the revision number. I suggest a solution which is quite similar to Mercurial's built-in web interface:

• Every link leads to a file at the currently shown revision instead of its history;
• There is a button at the commit screen to browse repository at this revision.