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

1. As an admin, go to /applications/view/PhabricatorDashboardApplication/
2. See only two options "Can Use Application: All Users" and "Can Configure Application: Administrators"
3. Apply patch
4. As an admin, go to /applications/view/PhabricatorDashboardApplication/ and see the new option "Can Create Dashboards" set to "All Users"
5. As an average user, go to /dashboard/ and successfully create a new Dashboard
6. As an admin, go to /applications/edit/PhabricatorDashboardApplication/ and change "Can Create Dashboards" from "All Users" to "Administrators", select "Save Policies" button
7. As an average user, go to /dashboard/ and see that "Create Dashboard" is disabled
8. As an average user, go to an existing dashboard not created by this user and see that selecting "Edit Dashboard" shows "You do not have permission to edit this object."
9. As an average user, go to the existing dashboard that you created yourself and see that the Edit Policy is set to this user.
10. As an average user, go to the existing dashboard that you created yourself and see that you can still create or add panels.