Persist password (#393)

* never hold the owner of the room on the waiting screen

* persist room passwords so we can always call create meeting, even if it's already running, to avoid any inconsistency or race condition when joining a meeting

* Fixed issues in migration, room attribute updates, random_password and tests

Gemfile.lock

@@ -215,6 +215,7 @@ GEM
       thor (>= 0.18.1, < 2.0)
     rainbow (3.0.0)
     rake (12.3.1)
+    random_password (0.1.1)
     rb-fsevent (0.10.3)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
@@ -355,6 +356,7 @@ DEPENDENCIES
   puma (~> 3.0)
   rails (~> 5.0.7)
   rails-controller-testing
+  random_password
   redcarpet
   redis (~> 3.0)
   rspec-rails (~> 3.7)