Added functionality to show content of a merge request and navigate the merge request diffs

[liechtir]

Hi I have added a toggle view also for merge requests, where the developer can see the files received from rest API of gitlab, and when clicking on it, the corresponding diff view is opened. This comes in very handy for big merge requests, as gitlab has issues displaying them. It is best if the user sets to show a single file in the diff view in his user preferences.

Issues that I could not resolve due to my limited knowhow of react and your project

1. the filter checkboxes at the bottom actually work in the background, however the state is not stored properly. If you could fix that, that be awsome.
2. Also, when moving between merge requests and repository browser, the tree is not always refreshed. Sometimes the same happens when moving between different merge requests, the tree display stale data. closing the tab and reopen it works around it.
3. The refresh of the window might be overkill if you know a better way of doing it.