Basic new user registration email, various improvements

Spawned from discussion in #2463

This MR adds a simplistic new user registration email to provide positive feedback to the user that their account was successfully registered. There have been situations in the past with FE and/or captcha issues that caused users to believe their account did not get registered, so they try again or with a new variant of their username e.g., user2. This will prevent that problem.

While here, standardize these functions on the maybe_ prefix and mark private if possible.

Additionally a slight improvement (IMO) to the language for confirmation emails has been made.

Plaintext versions of emails are now generated except for the Digest which will require more work Removed due to concerns of unknown quality output at the moment.