Make GITHUB_API on client configurable

[ktong]

Our GHE can be connect directly from browser. So if GITHUB_API is configurable, then we do not need server side proxy.

[ian-r-rose]

Do you connect with an access token? The primary reason for the server-side proxy is security: since the access token or client_secret are effectively passwords to a user's GitHub account, it is unsafe to store them client-side.

[ktong]

It does not need access token as user already accesses GHE. Probably Cookie enables access of GHE. If not, access token in access URL also acceptable.

[ian-r-rose]

I have no objection to this enhancement (so long as there is no security issue), but I can't really do it myself without good access to a GHE account. PRs welcome though, and I'm happy to help!