Forbid construction of dashboard panels on various Phortune objects

Summary:
Fixes T7102. These panels don't work, aren't meaningful, and don't seem very useful.

We could eventually support providing context to dashboards somehow ("merchant dashboard") but don't have much of an apparent need for this.

Test Plan:

• Tried to create cart/subscription/charge dashboard panels.
• Unable to create new ones.
• The ones from before the change show a relevant error now.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7102

Differential Revision: https://secure.phabricator.com/D11953