Assign multiple groups to each email report

[jniles]

We need to support multiple groups assigned to each email report. Currently, we only allow a single group assignment.

Basically, the following changes need to happen:

1. Instead of having a single User Group, we need to change it to a multiple user groups in the dropdown menu on the create page (<select multiple>).
2. Modify the email sending procedure to loop through groups, and create a master list of users who should be receiving the report. Then it will send them out as normal.