Added ability for the `field_agent` role.
Add abilities for the Supervisor role.
A field_agent should be able to generate excel for his own responses.
Show the 'Download Excel' button only if the current user has permission to download excel.
Don't show the `Active, Archived` etc. filters only if the user isn't logged in.
Add abilities for the `Designer` role.
Add abilities for the `Manager` role.
Add abilities for the `Admin` and `SuperAdmin` roles.
Allow deleting a survey only if it has zero responses.