[PD-767] Replace slow multi-select widget with FSM widget and lazily load those widgets

The othe two pull requests didn't end up being as intense as I thought, and seemed to be causing confusion. Doing it this way instead.

Changelog:
* replaced multiselect widget with FSM widgets
* created views and urls necessary to support said widgets
* removed the init for the reverse sitepackage form as it seemed superfluous. 
* lazily load contents of widget to reduce load time from (on my QC server) from 37 seconds to 5.