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.
• Tested name: valid, omitted.

Reviewers: btrahan, jungejason, davidreuss

Reviewed By: btrahan

CC: aran, btrahan

Differential Revision: https://secure.phabricator.com/D1365