"Enable Outgoing Mail" button on the Outgoing Mail admin page should be visible to regular admins, not just sysadmins

I've seen two cases now where the Outgoing Mail page has said that outgoing mail is disabled, but the big button to enable mail is not visible to regular admins, only sysadmins.

It should be visible for admins as well.

Because of how OnDemand vs BTF work we should:

• Create an option on the JIRA Mail Queue page or the General Config page for enabling / disabling mail
• Only show this option if the instance is an OnDemand instance, not for BTF

This prevents us revealing the mail config page in OD and prevents doubling up config options for BTF customers