djblets.auth.views.register() emits user_registered signal on success.

`djblets.auth.views.register()` emits `user_registered` signal on success. This
is useful for doing additional work outside of `djblets.auth.views.register()`
when a user registers.

Testing Done:
- All unit tests pass.
- Manually tested it, and it works.

Bugs closed: 3438

Reviewed at https://reviews.reviewboard.org/r/6334/