Refactor message group selector

• Store references to dom nodes in member variables to avoid querying them over and over again
• Refactor some code into separate functions for clarity
• Consistency and style improvements:
  • New line after dom node creation
  • groupSelector = this
  • use $.proxy when possible
  • use .then() to transform return values in promises
• Fixes a bug where filter was not applied if user was typing before groups were loaded
• Use 'one' for the event to close the menu to avoid extra processing

Change-Id: I8175a78e1becb3276550b360d6dbbd41b800de64