allow arcanist projects to be deleted via the ui

Summary: tricky part is purging symbols at that time too. override "delete" method to get there, use transactions, etc.

Test Plan: deleted an arcanist project - it worked!

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, mbishopim3

Maniphest Tasks: T1738

Differential Revision: https://secure.phabricator.com/D3613