Allow use of another Firebase

[cben]

Not really a solution to #4 but a nice easy suggestion by @tohagan that does improve privacy, and might simplify the legal situation with Firebase ToS by having other devs agree to them themselves.

It improves privacy because I have admin access to all user data under mathdown.firebaseIO.com (I've never used it to peek at other's texts but you can't know that. Muhaha...); opening your own firebase keeps access between you and Firebase (i.e. Google). [There is also the hosting server — on RHcloud in mathdown.net case — that sees secret doc IDs.]

It never occurred to me because I really like the fact that you can fork my code and still access — and collaborate — on the very same documents. But we can have it both ways: URL would include the firebase, e.g.

mathdown.net?firebase=yourbase&doc=docId

(I think the default if omitted should stay mathdown but that's not critical.)

The nice property is there are no Firebase keys anywhere in the source! The only auth now is knowing the secret docId; in the future there will be user accounts but still the auth will be directly between user and firebase.

Moreover, "having it both ways" also means being able to use your own firebase without fork/self-hosting the site (if you trust my web server but not me) => Once firebase=... is set, it should be used accross [New Document] creation, so all it'd take is registering a firebase and bookmarking mathdown.net?firebase=yourbase.

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As for the Firebase ToS aspect, it's hardly a solution because I expect most users — and even forkers — would use (at least some of the time) the main firebase.