Update Contributing Docs to include Developer tips
Merge pull request #1944 from b1nd/docs

Update Contributing Docs to include Developer tips
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.
Merge branch 'move-ids' into 0.4-maintenance