Dashboards - introduce ability to optionally allow SearchEngines to be used as dashboard panels

Summary:
Ref T7234. Turns out some search engines are context specific such that they can't be bubbled up to a dashboard panel generically. The example in question is an Instance Members search, where the instance must be specified and is done so in normal codepaths but the dashboard panel stuff has no way of doing that. Ergo, just turn off these sorts of panels.

Note this code just makes it so we can turn off these sorts of panels but does not do any of that.

Test Plan:
made sure all the queries still showed up

otherwise, next diff

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7234

Differential Revision: https://secure.phabricator.com/D11750