Once upon a time, reviewboard.account.backends just housed a couple of

small authentication backends, but over time the number of backends grew

and the infrastructure around it evolved, turning it into a pretty large

module.
This change splits up this module into smaller pieces, and adds

forwarding imports to ensure that all existing code works as before.

Each backend is now in its own module, with the base class being in a

base.py and all tracking of modules and settings in a registry.py.
Aside from code movement, there are two additional code changes:


AuthBackend (the base class) has been renamed to BaseAuthBackend.

   An alias exists in reviewboard.accounts.backends to prevent

   existing code from breaking.


INVALID_USERNAME_CHAR_REGEX is now a member of BaseAuthBackend,

   instead of being a global variable. An alias exists for this as well.


Documentation and any remaining code style improvements will be made in

an upcoming change.

Unit tests pass.
Ran Review Board and selected authentication backends. Didn't encounter

any issues loading a list of modules or their forms.
Tested that the old entrypoints (without re-running setup.py develop)

continued to work due to the forwarding imports.
Tested that the new entrypoints overrode the old ones and work.