feat(email): Add secondary email mail templates Part 4

[vbudhram]

Woo, part 4 for adding secondary emails, connects with #1672. This contains all the email templates needed, verifySecondaryEmail and postVerifySecondaryEmail. Designs for these emails are here.

@mozilla/fxa-devs r?

[philbooth]

Again, I think I'm of the opinion that this should be part of #1768, rather than add commits to the tree that knowingly contain TODO text for the templates. Not sure if anyone else agrees?

[vbudhram]

@philbooth I'm going to pull these into #1768. I think it might be ok because most of the file changes in this repo are generated from grunt templates.