Convert review group's mailing list to a generic CharField.

Our email backend allows the mailing list field to contain multiple e-mail
addresses separated by commas. Unfortunately, because this was represented as
an EmailField, the validation in the admin UI didn't allow entering multiple
addresses. This change converts the field to be a generic CharField.

Testing done:
* Ran evolution
* Ran unit tests
* Added a group with multiple mailing lists.

Fixes bug 1661.

Reviewed at https://reviews.reviewboard.org/r/5305/