Listen to firebase events to change the list of active editors

[shashaBot]

Epic: #4

Related: #16 , #6

Further build upon #6 to complete the callback functions for firebase events child_added and child_removed to dynamically add/remove avatar to/from the avatar list (#16 ).

Please ensure that the current user adds/removes themselves to/from active editors list in firebase database. User should remove themself when they close the tab (need to listen to browser event to capture this).