Disable internal issue tracker if external tracker in use

### What does this MR do?

This MR disables routes to the internal issue tracker if an external issue tracker (e.g. Redmine) is in use. A 404 page is returned if a user attempts to access any of the issue routes.

### Why was this MR needed?

Enabling the external issue tracker would still keep the internal one active, which would lead to odd behavior where users could create issues but not close them.

### What are the relevant issue numbers?

Closes #2006

See merge request !993