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 reset/welcome emails are silly.

• This does not disable the "fax these changes" message in Arcanist (no

reasonable way for it to read the config value) or the rainbow syntax
highlighter (already removable though configuration).

Reviewers: moskov, jungejason, nh, tuomaspelkonen, aran

Reviewed By: moskov

CC: aran, moskov

Differential Revision: 1081