Add self-service group verification

[tobias]

Fix typo

Add util for self-verification of groups

Add self-service TXT-based group verification

This will allow a user to self-verify a group that is reverse-domain based by setting the correct DNS TXT record.

Add util for self-verification of subgroups

Add self-service parent-based group verification

This will allow a user to self-verify a group that is a subgroup of an already verified group.

Add util for self-verification of VCS-based groups

Refactor common verification logic

Add self-service VCS-based group verification

This will allow a user to self-verify groups that are based on a VCS organization.

Email admins for group verification attempts

This will allow us to track verifications for a bit to see how the system is working.

This implements #853.

[tobias]

Here are some screenshots of the feature. All three of the verification forms are on one page.

Here is each form with the help expanded:

Here are some of the error responses:

And, finally, some successful responses:

[tobias]

Hi @danielcompton - I've updated this PR based on your feedback. Do you mind taking another look? Thanks!