Forbid mailing list names contianing spaces or commas
Summary:
They end up in "CCs:" fields where they can't be parsed.
Not bothering to migrate since I think only Dropbox has hit this.
Also improved another error condition's handling.
Test Plan: Tried to save a mailing list with spaces and commas in the name.
Reviewers: btrahan, Makinde
Reviewed By: btrahan
CC: aran, epriestley
Maniphest Tasks: T947
Differential Revision: https://secure.phabricator.com/D1813