R1:65a56c6ce092

Improve mailing list edit form

Summary:
- Add some captions to make it more clear what these fields mean.
- Require "name", since tokenizers use it exclusively.
- Limit URI to allowed protocols, since admins can currently XSS users by
entering a "javascript:" URI and then tricking the user into clicking the
mailing list name. This exploit is dumb, but technically privilege escallation.

Test Plan:
- Created a new mailing list.
- Edited a mailing list.
- Tested URI: valid, invalid, omitted.