Preserve a list of permanently banned email addresses to prevent abuse via reusing after account is purged

[jdavcs]

Based on a recent discussion on galaxyadmin slack: it would be nice to have a way to permanently ban users and save the banned email addresses to prevent them from being reused for registering new accounts after the banned accounts have been purged.

This could be a per-galaxy list or one optionally-available master list, or both. It can be implemented as a database table (then we can store additional relevant information (banned when and why), or as a simple text file (we can hash the email addresses so they are not made public).

@natefoo @jennaj

[ElectronicBlueberry]

just as a note: let's not forget plus addressing if we implement something like this

[natefoo]

Gmail dots as well, but hopefully there aren't a million special cases like Gmail.