R1:b571f0b229be

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