Consolidate email methods

[squidgetx]

From #347, it would be nice to have a method that simply can send an email based on the status of the reservation (checkin, checkout, overdue checkin specifically)

[orenyk]

This sounds like a strong improvement to our current code; that said, I can't figure out when we're calling these methods - it seems like they were supposed to be implemented in buttons on various pages but they're all commented out. If we're not using these e-mails then we should simply remove them (although having a single reservation status-based e-mail method is probably useful). Moving to v4.2 for now, but we should discuss.

[orenyk]

These were disabled over the summer; looking into why that was...

[squidgetx]

I don't remember ever disabling them, IIRC they were already disabled when we started.

There might have been something wrong with the button but I can't easily see what that would be...

[orenyk]

Git blame to the rescue, it was @dgoerger here... waiting to figure out what was going on.

[dgoerger]

The buttons were commented out during the Great Rewrite of 2012, when clicking them resulted in UNDEFINED METHOD and similar errors. If the methods exist and work as expected now, great, but they certainly didn't in 2012. :)