validators: remove unneeded M/StateObj classes

The M class is actually a wrapper around formencode.api.Validator.message,
inserting a translator into the 'state' object. Setting a translator into
the state object is indeed what is mentioned in the formencode docs.

But, if you work this way, the custom state should be set both for custom
validators, as well as for validators that are simple wrappers around
formencode's default validators (for example wrappers that just set a custom
message string). And the latter is what Kallithea is currently _not_ doing.

Also, when using formencode.api.Validator.message correctly, you should not
use the translator function _ on your validator strings manually.

Remove the inconsistency in Kallithea validators as follows:
- remove M and StateObj classes
- replace the usage of M by direct calls to self.message (which is now no
longer in charge of translating but simply of mapping a keyword onto the
right message)
- translation will be done by explicit _ calls at Kallithea side, so that
formencode directly receives translated strings.