Migrate from SRP to bcrypt.

See the spec https://meteor.hackpad.com/SRP-bcrypt-J5mdBojeVfe for
details.
account data is no longer used for the srp challenge
code review items
code review II
Nick code review items (mostly comments)
Make an error message nicer
Remove 'plaintextPassword'.

'password' can always be either a string or an object (indicating that
it's been hashed already with something). When the server receives a
string, it hashes it with SHA256 before bcrypt.
Extract number of bcrypt rounds to top of file
Merge branch 'srp-to-bcrypt' into devel