Add async job indicator when saving changes in the 'Manage Repositories' page

Created by: arussellsaw

fixes https://github.com/sourcegraph/sourcegraph/issues/16948

I also have some feedback from @felixfbecker on general code quality that i'll fix in another PR

I had to add custom styles for the loading spinner here, as our default spinner uses the wrong colours for the designs, and for some reason the builtin bootstrap spinner doesn't work 🤔 . If there's a better approach please let me know!

i also fixed some whitespace issues, so sorry for the large diff on this, probably worth reviewing with 'hide whitespace changes'

https://user-images.githubusercontent.com/5236823/104449583-b7151680-5596-11eb-8124-a55ef93fae43.mov