feat: Document presence indicator (#1114)

* Update websockets to allow joining document-based rooms

* dynamic websocket joining

* emit user.join/leave events when entering and exiting document rooms

* presence storage

* feat: frontend presence store

* lint

* UI updates

* First pass editing state

* refactoring

* Timeout per user/doc
lint

* Document data loading refactor to keep Socket mounted

* restore: Mark as viewed functionality
Add display of 'you' to collaborators

* fix: Socket/document remount when document slug changes due to title change

* Revert unneccessary package update

* Move editing ping interval to a shared constant

* fix: Flash of sidebar when loading page directly on editing mode

* separate document and revision loading

* add comments for socket events

* fix: Socket events getting bound multiple times on reconnect

* fix: Clear client side presence state on disconnect

* fix: Don't ignore server side error
Improved documentation

* More comments / why comments

* rename Socket -> SocketPresence

* fix: Handle redis is down
remove unneccessary join

* fix: PR feedback

server/migrations/20191228031525-edit-presence.js

@@ -0,0 +1,14 @@
+'use strict';
+
+module.exports = {
+  up: async (queryInterface, Sequelize) => {
+    await queryInterface.addColumn('views', 'lastEditingAt', {
+      type: Sequelize.DATE,
+      allowNull: true,
+    });
+  },
+
+  down: async (queryInterface, Sequelize) => {
+    await queryInterface.removeColumn('views', 'lastEditingAt');
+  }
+};