Simplify registration page copy

Because we no longer offer the "Forgot Password" form, the only function
of the page is registration. The page title is updated to reflect this
and the "New MDN Members" heading is removed.

Other changes are made, mostly minor. The Bugzilla link is removed -- we
only receive about one report per month, so it seems unnecessary to
burden all users with this notice. People who deeply care about getting
their old accounts back will likely submit a bug with or without this
notice.
Merge pull request #2506 from openjck/register-copy

Simplify registration page copy