Process wiki page updates in the back-end

[reinierl]

As a CyclePlanet admin, I want to be able to restore a wiki page after it's been vandalised.

As a CyclePlanet admin, I want to be able to trace down the author of seriously objectionable content.

As a CyclePlanet member, I want to be able to find the author of information in order to critically review it in order to improve it.

To get there, we have to:

• Make wiki pages not writable in Firebase except by cloud functions
• All updates must go through a cloud function that writes the new content to Firebase
• The cloud function makes sure we store the author and timestamp of the new version
• The cloud function also makes sure we store the old version, or a diff between the old and the new
• Add a UI to view history and revert edits (by creating a new version with the old content, not throwing away the newer bit of history!)