Add optional/configurable feedback module

Most sysops didn't know it, but if exec/feedback.* existed, it would be
executed just before any user sent an email to the sysop (user #1),
excluding new user validation requests:
- make this module name configurable and loadable from mods
- support JS module here (exit(1) to abort the feedback)
- invoke for email being sent to *any* sysop (not just user #1)
- don't invoke the module when sending *from* a sysop account

This fixes issue #16