Add checkbox to make notice of comment optional

[Karl-Isac]

Now everytime someone comments on a Utlägg the OP is notified via email. This is not neccessary for example "X har attesterat ditt utlägg". So it would be better if you could choose when to notify of more imortant comments.

[Lulcaz]

Emails for attest, confirmation, payment and accounting of expenses should probably be opt-in since these aren't aking the expense-holder to do something.