Change sign-up form styling from an ID to a class. Fixes #1605

Move styling from some elements from IDs to classes

Closes #1605

* Move styling for `#signup`, 	`#forgotten`, `#reset`, `#login`, `#notifications` and `#modal-container` to classes
* No IDs have been added or removed, so any events shouldn't be affected
* Introduces 6 new classes `signup-form`, `forgotten-form`, `reset-form`, `login-form`, `notifications` and `modal-container`