Add protocol handler. Handle follow intents (#4511)

* Add protocol handler. Handle follow intents

* Add share intent

* Improve code in intents controller

* Adjust share form CSS

app/controllers/intents_controller.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class IntentsController < ApplicationController
+  def show
+    uri = Addressable::URI.parse(params[:uri])
+
+    if uri.scheme == 'web+mastodon'
+      case uri.host
+      when 'follow'
+        return redirect_to authorize_follow_path(acct: uri.query_values['uri'].gsub(/\Aacct:/, ''))
+      when 'share'
+        return redirect_to share_path(text: uri.query_values['text'])
+      end
+    end
+
+    not_found
+  end
+end