Make final design changes

* Checked icon now SVG
* Made auto-enroll an HTMLTextBlockField (new class)
* Removed screenshot for auto-enroll
* Changed some l10n strings to conform to design
* Added HR to form (another new class)
* Shifted things around with margin/padding

Also added tests for the new HTMLFormField subclasses.

Resubmission of reverted 52a58b316d.

Change-Id: I8db96bbeaba58e3c6ac8c136137c3cf9573ee86b