bug: Update fetches_things.rb to use custom policy if available

[ddmck]

Description

Fixes #1955

If self.authorization_policy is set on a resource and the current user would pass the check for index? the resource does not show up in the navigation menu.

Updating the Services::AuthorizationService.authorize call in Avo::Concerns::FetchesThings#get_available_resources appears to resolve this.

No tests are failing locally, but I still need to figure out the test suite so that I can test this functionality

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Manual review steps

1. Step 1
2. Step 2

Manual reviewer: please leave a comment with output from the test if that's the case.

[codeclimate]

Code Climate has analyzed commit ec6bc71 and detected 0 issues on this pull request.

View more on Code Climate.

[Paul-Bob]

Thanks for the investigation and for the PR @ddmck! I made a change suggestion, let me know if it make sense