R1:fbfb263cd9ef

Provide a configuration flag to disable silliness in the UI

Summary: See comments. A few installs have remarked that their organizations
would prefer buttons labled "Submit" to buttons labeled "Clowncopterize".

Test Plan:
- In "serious" mode, verified Differential and Maniphest have serious strings,
tasks can not be closed out of spite, and reset/welcome emails are extremely
serious.
- In unserious mode, verified Differential and Maniphest have normal strings,
tasks can be closed out of spite, and…