Fix backend sharing bug

[jdavcs]

This is part of decoupling role names from user email addresses.

Bug: after a user's email is modified, sharing roles cannot be retrieved for that user:

• role is initially created with name=f"Sharing role for: {', '.join(u.email for u in users)}"
• ...and retrieved based on (Role.name).like(f"Sharing role for: %{user.email}%")

The solution here is to retrieve based on the association defined via the UserRoleAssociation model.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Run the new unit test without the commit with the fix
  2. Apply commit with the fix and rerun the test.

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.