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

@@ -111,3 +111,5 @@ end
 gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
 
 gem 'coveralls', require: false
+
+gem 'random_password'