Workboards - add new "initialization" flow

Summary: Currently, we just create a default "backlog" column if / when you visit a workboard for the first time. Post this patch, instead you see a blocking dialog that lets you either create the default backlog column or import columns from another project. In the case of the latter, the user gets another dialog which lets them select any project of which they are a member that also has columns in it. Note that only not hidden columns get imported. Fixes T4431.

Test Plan:

• made a new workboard and got my new dialog. made a default backlog and it worked!
• made a new workboard again and tried the import flow - it also worked.
• verified projects with no columns do not show up in import dialog
• verified project with / without columns still all show up in maniphest project typeahead

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4431

Differential Revision: https://secure.phabricator.com/D10153