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/services/verify_link_service.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+class VerifyLinkService < BaseService
+  def call(field)
+    @link_back = ActivityPub::TagManager.instance.url_for(field.account)
+    @url       = field.value
+
+    perform_request!
+
+    return unless link_back_present?
+
+    field.mark_verified!
+    field.account.save!
+  rescue HTTP::Error, Addressable::URI::InvalidURIError, Mastodon::HostValidationError, Mastodon::LengthValidationError => e
+    Rails.logger.debug "Error fetching link #{@url}: #{e}"
+    nil
+  end
+
+  private
+
+  def perform_request!
+    @body = Request.new(:get, @url).add_headers('Accept' => 'text/html').perform do |res|
+      res.code != 200 ? nil : res.body_with_limit
+    end
+  end
+
+  def link_back_present?
+    return false if @body.empty?
+
+    Nokogiri::HTML(@body).xpath('//a[@rel="me"]|//link[@rel="me"]').any? { |link| link['href'] == @link_back }
+  end
+end