Ensure the unsupported_dialog and cookies_disabled screens show without the loading screen.

Ensure the styling of the unsupported_dialog and cookies_disabled screens match other parts of the site. (vertically centered error messages)

fixes #3783
fixes #3784
Fix Fennec Nightly's bottom margin for the unsupported dialog.

Only used position:absolute on the unsupported dialog in mobile.
Merge pull request #3785 from shane-tomlinson/issue_3783-unsupported_loading

Ensure the unsupported_dialog and cookies_disabled screens show without the loading screen.