working indicator

Where is the indicator used?:
- on batch update.
- on every attempt to fetch a collection. To see it you would need to set bootstrapped data to false in /public/app/browseWorks.js : 146;
- while saving an edited work
This mixin already tracks what happens during the save progress to enable/disable buttons, so rather than acting on the indirect events to show the working indicator, it would be better to just integrate showing/hiding the working indicator in the existing functions.