Conpherence + workboards - fix dragging behavior breaking on subsequent page loads

Summary: Fixes T8155. The former "init_board()" function was aptly named as it needs to be called each time a new board dom piece is downloaded. Ergo, break out a setup() function and call that in the once-only setup place, and use init_board() there as well as when we have a quicksand redraw event with data from the server.

Test Plan: paged about a project and was able to keep dragging and dropping tasks on various loads of the board. verified drops saved correctly from load to load.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8155

Differential Revision: https://secure.phabricator.com/D12798