Scrollable File/Source view
### Description
It would be desirable to allow scrolling within the file source view window (instead of using the browsers scrollbar) so that *only* the content within the source viewport scrolls - rather than surrounding elements such as the branch selector and Raw/Blame/History/Permalink/Edit/Replace/Delete buttons - because I regularly find myself in the situation where I'm viewing the content of a large source file and then want to quickly switch branch, or switch to Raw/Blame/History views - which means I need to manually scroll back up to the top of the page to view these UI elements.
GitLab CE version at the time of writing: 8.11.x
### Proposal
Two immediate solutions come to mind:
* make the content of the source view independently scrollable (perhaps something similar to the scrollbar which currently appears in the File 'Edit' view)
* make the branch selector and Raw/Blame/History/etc buttons 'sticky' so that they are always viewable (or at least 'pinnable') even when you scroll down through the source code...
### Links / references
issue