Misc updates for registration

[katjuell]

Description

Misc updates for user registration, including error messages for emails, and mobile styles on to-dos/don'ts.

Test process

Profile changes look reasonable on mobile etc.

Users should get a warning if they haven't selected an email during the registration process.

This should also resolve the $video errors in Airbrake.

Requirements to merge

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code in hard-to-understand areas
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes