Provide better support for "announcement" service desks that are read only

Some customers may wish to utilise a read only service desk (where customers cannot raise issues) and have agents raise issues to be shared with various organisations.

This is currently possible to implement now (by removing the "Create Issue" from the "Customer Portal Access" group - although you'll run into this bug).

Even if the bug were to be fixed, the portal isn't necessarily designed for a read only state. It'd be great to be able to mark a Service Desk Project as a "Read Only for Customers" project, which would cover a number of things:

1. When you remove the "Customer Portal Access" group from the Create Issue Permission, Jira will alert you saying that the permission scheme has errors (but it's perfectly valid for this use case)
2. The portal view should let users know that they can't create a request, but can see requests shared with them or their organisations (and link to the appropriate views)
3. The portal view should also make it clearer that if they do need to raise a request, they can visit the other help desks in the system (and should refer them back to the list of Service Desks).