R1:21dca29c5f29

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.