Restore the use of passwords.Schemes enum for selection of password hashing

scheme.  Fix mmsitepass and test cases accordingly.  Details:

- set_global_password(): Instead of taking a string for 'scheme' argument,
  take None and then coerce that into passwords.Schemes.ssha

- Add a base PasswordError and a BadPasswordSchemeError error that derives
  from that.  For consistency, multiply inherit MMBadPasswordError and
  MMPasswordsMustMatch from PasswordError.

- Add a passwords.lookup_scheme() method which turns scheme_names into scheme
  enum constants.  It returns None if the lookup fails.

- passwords.py: change the internal representation of _SCHEMES_BY_TAG
  dictionary to map scheme names to scheme enum values.  Change internal uses
  of this dictionary to then turn those enum values into hash classes, or
  whatever else we need.

- make_secret(): Raise BadPasswordSchemeErrorif the given schema (which should
  be an enum value) is invalid.

- TestBase.tearDown(): Clear out any <site> locks that might hang around after
  a test case runs.

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