Validate menu item fields (links, projects, dashboards, applications, forms, etc)

Summary:
Ref T12128. This adds validation to menu items.

This feels a touch flimsy-ish (kind of copy/paste heavy?) but maybe it can be cleaned up a bit once some similar lightweight modular item types (build steps in Harbormaster, blueprints in Drydock) convert.

Test Plan:

• Tried to create each item with errors (no dashboard, no project, etc). Got appropriate form errors.
• Created valid items of each type.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12128

Differential Revision: https://secure.phabricator.com/D17235