Merge branch '20621-avoid-lame-conflicts-in-ability-allowed' into 'master'

Move abilities by subject class to a dedicated method

## What does this MR do?

Twin of gitlab-org/gitlab-ee!622.

It moves the `case` that returns abilities depending on the subject class to a dedicated method.

This should avoid lame conflicts when CE to EE since the EE version assign the result of the `case` to a variable.

## What are the relevant issue numbers?

Closes #20621.

See merge request !5668