Add navigation (previous/next version) buttons in the changes view

Using the changes view to hunt down when a particular change was done is pretty painful. The new Blame View sure helps, but for things like deletions there is no other option but to still go through individual changes and find what you are looking for.

Right now, the pain comes from the 4+ clicks you need to perform in order to see the next change:

• Click back in the browser
• Click the new previous version radio button
• Click the new next version radio button
• Click Compare versions
• (Repeat N times until change is found)
• (Suffer even more when previous version is on one page and next version is on the next)

To avoid all this, we should add 2 simple back and forward buttons to the left and to the right of the compared versions in the changes view to allow easy navigation between changes (consecutive version pairs).