Change "insane" to "absurd" (referring to high fees) in text strings and identifiers.

Note that this will also require translation changes in Transifex for the key
"A fee higher than %1 is considered an insanely high fee." which is now
"A fee higher than %1 is considered an absurdly high fee."

Signed-off-by: Daira Hopwood <daira@jacaranda.org>