Rename "BrowserID" --> "Persona"

Note that this invalidates all saved persistent replications that used Persona/BrowserID auth,
because the property in the replication doc was renamed too.
It also changes the server endpoint that the replicator hits to log in from /db/browserid to /db/persona.

(This is equivalent to commit 111859c from CouchbaseLite, although it's not a direct cherry-pick. I had to make the same changes manually because too many things have been renamed for a merge to work.)