Minor /main layout changes
Added ng-maxlength to displayName on /account

This display name limit (of 20 characters) is the same as the limit on
/signup
/signup now shows previous email and display name

When there's an error on the /signup page, the user will be shown the
same page without password info.
Added html-escaping throughout app

As it turns out, ERB does not automatically escape HTML symbols in
strings. This misunderstanding led to several issues where HTML could be
injected into a page.