Fix #5578

UX-wise, people expect that saving the profile will re-check links even without changing fields content. Bug-wise, `@account` was undefined.

Regression from #8703

* Verify link ownership with rel="me"

* Add explanation about verification to UI

* Perform link verifications

* Add click-to-copy widget for verification HTML

* Redesign edit profile page

* Redesign forms

* Improve responsive design of settings pages

* Restore landing page sign-up form

* Fix typo

* Support <link> tags, add spec

* Fix links not being verified on first discovery and passive updates