Disallow managing roles with greater privileges

www.agileplannerapp.com/boards/173808/cards/5254

* Only allow superadmins to view and manage other superadmins
* 'Filter by Role' only shows roles that current user can manage
Clean up styling on change email form

Change email form was missing default Bootstrap classes, meaning
disabled fields didn’t look disabled, labels weren’t above fields and
fields were the wrong size.

* Add bootstrap classes
* Keep page title and h1 consistent
* Move warning message out of well to avoid nesting
* Stop nested labels around passphrase field
Merge pull request #274 from alphagov/change-email-style-fix

Clean up logged-out change email form
Merge pull request #273 from alphagov/hide-superadmins-from-admins

Disallow managing roles with greater privileges