Construct the thanks URI dynamically based on env.
Add a "home" link to the thanks page.

The top left header image isn't super obvious.
Merge branch 'dev.webcompat.com'

* dev.webcompat.com:
  Very small edits/fixes
  Very small edits/fixes
Merge branch 'dev.webcompat.com'

* dev.webcompat.com:
  Change of opacity on submit buttons and make disabled on empty field
  Added 'required' attribute to required form fields
Closes #137. Rename form variable to bug_form to be less confusing.