Refactor AccountActivationMailer and SendConfirmationEmailController

[slkrx]

Fixes #2193

Changes proposed in this pull request:

Moves the common code between SendConfirmationEmailController and AccountActivationMailer into a trait.

Reviewers should focus on:

I wasn't sure what to name the trait or where to put it, but because most of the code for the trait was taken from the AccountActivationMailer class I decided to name it the AccountActivationMailerTrait and keep it in the same directory. Also, I decoupled the generateToken and getEmailData functions because I think it will make more modular and maintainable even through it may make using the trait more verbose.

Confirmed

• Backend changes: tests are green (run composer test).

[askvortsov1]

Looks good to me! We're currently in QA for beta 15, and there's a holiday break scheduled after that release, but I see no reason why we shouldn't be able to get this in for beta 16. Thanks!

[askvortsov1]

A side effect of this would implement #2515, which still needs a bit more discussion before merging this. That being said, I don't think anything should be changed here yet (no need to exclude the prefix conditionally), unless a decision is reached to keep the prefix in one place but not the other, which seems kinda unlikely.