Make diff file view easier to use on mobile screens

Viewing diffs on a mobile screen is a bit of an awkward experience at the moment. Here are a few issues (by no means complete):

## Before

### Tabs are scrunched

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/45924e36cfb107dc56795a7dcdfbc5b5/image.png)

### Filenames take too much room, buttons cluttered

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/a5e5611271fe26dfd3922aa0b8780458/image.png)

## After

This MR makes a few tweaks to make this a bit better. It just addresses a few issues, but there is plenty of room for improvement (e.g. shrink fonts, etc.):

### Eliminate padding to make tabs fit

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/fbcf128d1f221b9b9e53df274238dc62/image.png)

### Make filenames, buttons more readable

This screenshot allows the filename to use the whole row, omits the file mode changes, and puts the buttons centered in the view:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d2e5b9424f058827ba2552056c516e18/image.png)

Towards a better mobile experience: #2787 

See merge request !1449