registry: validate major version suffix strings on new modules

[shoenig]

Currently it's possible (and recurring) for people to incorrectly add something like
example.com/foo/bar v2.2.2
when they really intended to add
example.com/foo/bar/v2 v2.2.2
in the webUI. We should be able to parse both halves of the module coordinates and apply some rules to see if it's a nonsensical thing to add.