Cleanup: Partial code duplication related to finding current user's project membership

# Problem

The following files have a minor amount of code duplication:

```
components/project-details.js
abilities/task.js
abilities/project.js
```

The code is not identical in all 3 cases, but is quite similar and is related to finding the current user's membership in a project.

It's not urgent, but we should look into ways of eliminating it.

Depending on how we deal with #1150, this might not end up being an issue at all.