R1:1c59b6542136

Dashboards: add capability who can create Dashboards

Summary:
Add a `dashboard.create` capability to allow limiting dashboard creation (and creation of related panels).
This can reduce spam in open Phorge installations in which anyone can create an account while you still want anyone to be able to view existing dashboards.

Closes T15438

Test Plan:
1. As an admin, go to `/applications/view/PhabricatorDashboardApplication/`
2. See only two options "Can Use Application: All Users" and "Can Configure…