FPRule: Cleanup

FPRuleForm should display all commands except event detail and validation,
and will typically want to display new command usages. To make it easier to
add purposes/usages, make this a blacklist instead of a whitelist.  Since
all purposes should be displayed, do not modify that in the constructor.

Sorted FPRule purposes and FPCommand usages alphabetically.
FPR: Add validation rule purpose & command usage

refs #7216