Move styling from some elements from IDs to classes

Closes #1605

* Move styling for `#signup`, `#forgotten`, `#reset`, `#login`, `#usermenu` and `#notifications` to classes

No IDs have been added or removed, so any events shouldn't be affected and it passes all tests.

Change user menu ID to class
Merge branch 'move-styles' of https://github.com/PaulAdamDavis/Ghost into move-styles