Visual Styles

This is definitely still a work in progress, but it is looking like _something_.

Outstanding items:

- [x] Decide on vertical centering (thinking of not doing it)
- [x] Add Firefox logo to footer
- [x] Determine responsive breakpoints
- [x] Confirm linear gradient background parameters
- [x] Add vendor prefixes to CSS rules for maximum compatibility.
- [x] Add hover and active styles for buttons
- [x] Specifically style placeholder text

To-do later:

- Style selects
- Style error messages
- Determine strategy for "retina" images
- Add hover and active styles for links
- DRY up views using partials
- about:accounts improvements

No longer valid due to design/flow changes:

- ~~Rework errors on `complete_sign_up` so that the title is correct~~
- ~~Decide how to style back buttons~~