Add option to resend an email receipt

[iannuttall]

This adds an option to the Edit Attendee page to allow organisers to resend an email receipt, as suggested by @alxblock in #123.

To test

1. Click an attendee from Tickets > Attendees
2. Click the "Re-send receipt" link
3. Assert that the correct receipt is emailed and that the meta log is updated

cc @iandunn

[iandunn]

Wow, I'd forgotten how complex and tightly coupled email_tickets() is; you're a saint just for touching this :)

I've spent a lot of time looking at it, and I think we may need some more drastic refactoring of email_tickets() first, so that we have a more flexible base to add this feature to.

I'm still playing around with a few different approaches, though. I'll pick it up again tomorrow.

[iannuttall]

Yeah, I felt like refactoring it any further was a task beyond me, but it was definitely interesting to delve into :D

Let me know if there's anything else I can do to help out.

[iandunn]

Ugh, I'm having trouble coming up with a decent solution; everything I try just becomes a rabbit hole.

@kovshenin, do you mind taking a look at this? Maybe you'll see a solution I don't.

[iandunn]

Sorry for the stagnation here. Given the difficulty with refactoring for a proper solution, I think it'd be ok to just allow the duplicate code. We should add notes to both functions to warn devs that they need to be kept in sync, though.

Can you rebase the PR against master so there won't be any merge conflicts?