Redesign forms, verify link ownership with rel="me" (#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

app/serializers/rest/account_serializer.rb

@@ -13,6 +13,10 @@ class REST::AccountSerializer < ActiveModel::Serializer
   class FieldSerializer < ActiveModel::Serializer
     attributes :name, :value
 
+    attribute :verified_at, if: :verifiable?
+
+    delegate :verifiable?, to: :object
+
     def value
       Formatter.instance.format_field(object.account, object.value)
     end