GHE Server support

[davegaeddert]

Everything so far should work for self-hosted GHE/PullApprove, except for the domain restriction:

pullapprove-browser-extension/manifest.json

Line 17 in 3b9a1d2

"https://github.com/*"

Matching all domains/pages is the easiest option, but seems pretty aggressive. Listing specific customer domains is not a good option either. I don't think a pattern match like https://*.github.* is possible.

There is also the question of whether self-hosted enterprise customers actually want an extension that is updated automatically, which is not how the self-hosted product works in the first place.