Improve names for 3 task actions-related functions

[lowellrex]

With the completion of PR #9014 we now have 3 functions related to task-action availability on the Task model named actions_allowable?, actions_available?, and available_actions. It is not immediately clear at a glance how those are different and what they do. This ticket exists to clarify those functions' purpose by (at a minimum) renaming them to be clearer. If during the process of the renaming we discover that there should be an accompanying refactor, we'll consider that as well.

Acceptance criteria

• Names for these 3 functions more clearly communicate their... function

[hschallhorn]

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Could be a larger ticket as these three functions appear in a lot of files. Coming to a consensus on the naming may prove to be a bigger lift than a simple find and replace. Renaming may create a refactor. After a very fruitful discussion on the reasoning behind higher estimations, votes were change to address these concerns.