Add support of github enterprise

[jianwu]

For many snippets that is work related. We can't store them in public github. So if this can store enterprise github will be great.

[SidneyNemzer]

Note that you can create a "secret" gist which is not listed publicly. However I think anyone can access it with the link, which understandably isn't acceptable for enterprise code.

I've never used GitHub enterprise but I assume it provides separate Gist storage? If so, we'd just need to replace github.com in a few places with a configurable hostname.

Looks like these URLs need to use the configurable hostname:

snippets/src/editor/constants.js

Lines 9 to 13 in a5cab28

	export const CREATE_ACCESS_TOKEN_URL =
	"https://github.com/settings/tokens/new?description=Snippets%20Access%20Token&scopes=gist";
	export const SNIPPETS_ISSUES_URL =
	"https://github.com/SidneyNemzer/snippets/issues/new";

snippets/src/editor/components/SelectGist.js

Line 10 in a5cab28

const usersGists = "https://gist.github.com/";

And then set the baseUrl for octokit: https://github.com/octokit/core.js#options

This would need to be another item in the settings page, and probably available as an option on the Login page too, hidden behind a button that says "Using GitHub Enterprise?".

I don't have time to dedicate to snippets right now, but I would accept a PR if you or someone else has time to implement this. I also don't have access to a GitHub Enterprise server so I would need help testing the changes.