Allow signature request to skip email

[ericberman]

if the instructor is really good about checking their students' flights awaiting signature, then can cut down on spam that way

[ericberman]

Or, don't send any email right away, but send one email at night to any instructor with pending requests. Need to keep track of how frequently they do this, though. Or make that an option on the instructor side: send me email when my student requests a signature (default), or send me email whenever I have pending requests (nightly).