Enables affiliate access to Zendesk

Gets things set up for enabling Canada Zendesk (#3322)

* Allows for a global `dosomething_zendesk_group_id` variable to be set.  If this is set, route all Zendesk tickets to this group.  We'll set this variable for Canada.

* Removes the check for affiliate in rendering the Zendesk form.  

This means we'll need to disable / uninstall the Dosomething Zendesk module on every affiliate before we push this code (blocked by #3349).

Once all affiliate sites have Zendesk uninstalled and this code is pushed, I will manually enable DoSomething Zendesk module on Canada, set up its auth variables, and set its `dosomething_zendesk_group_id` to the Canada Zendesk group.