Batch-admit users by email via the admin interface

[turbomaze]

• often times, accepting users is a team decision
• this may involve a separate review process that involves recording acceptance decisions in a separate medium (such as Google sheets or a custom webapp for application reading)
• the result of such review processes is a list of emails to accept
• currently, to automatically accept those users, the best thing you could do was write a short script that uses the reg API to admit each of them one at a time
• this is inconvenient and not user friendly
• to simplify this common use case, the admin page should support admitting whole lists of email addresses at a time

[DaleMitchell]

Bump! We're about to admit users for our event, MakeOHI/O. We would greatly appreciate if someone could make a pull request to close this issue.