Add option to disable default cron output email in tweak settings

Add an option in WHM to select where cron should send output by default. Options would include:

(a) To user's default mailbox (current default)

(b) Discard email (recommended)

If set to (b) the following line would be prepended to the resulting cron file (or updating existing line if it's already there):

MAILTO=""

From my experience I'd say 99% of crons being set up don't want their results sent to the user's default inbox, which is rarely monitored either way, and only creates useless load, inodes, disk space usage.

Easy feature that could be quickly implemented with little overhead and immediate positive results.