feature: Stronger email validation to avoid spam

[mjavason]

Description

Site owners never want dummy or disposable emails to end up on their sites. They always make complaints about getting spammed with useless emails. You could make the argument that verification solves this, but why not prevent them from registering in the first place. Saves everyone the headache.

Proposed solution

Two extra email validators. One for disposable emails and one for DNS/MX records.