Issue Summary

Issue priority scheme allows to associate with multiple projects making deletion of the priority scheme difficult

Steps to Reproduce

1. By default, all the projects are associated with the Default priority scheme
2. Create a new priority scheme Scheme 1 and associate a project Acme with it.
3. You will see that project Acme should be automatically disassociated from the Default priority scheme once it is associated with Scheme 1.
4. Go to Default priority scheme and add project Acme. Now you have 2 schemes associated with project Acme.
5. If you go to issue Acme-111 it will start displaying the priorities from the Default and not from Scheme 1
6. Try to delete Scheme 1 by disassociating the project Acme 

Expected Results

1. It should not allow priority scheme to be associated with 2 projects
2. It should allow to the disassociation of the project from a scheme (even if there are 2 schemes the project is associated with) and allow to delete the scheme

Actual Results

1. It allows one to associate projects with multiple priority schemes
2. It does not allow you to delete the scheme if point 1 is true. 

Workaround

The current workaround is to disassociate the project using the Priority scheme REST API . Once the project is disassociated you can delete the scheme.