Cleanup getProjectsForUser function

[lindapaiste]

Progress on issue #521

Changes:

• Combine common logic from getProjectsForUser (internal API) and apiGetProjectsForUser (public API).
• Add comments explaining the difference between the two handlers.
• Fix the "callback hell" by converting to async/await syntax.
• Update the test file to support async user lookup.
• Behavior change - internal API returns an error if there is no req.params.username instead of returning an empty array.

Note: there is a lot of discussion in PR #973 about wrapping an entire Express handler in a try/catch like I am doing here and whether there are better alternatives. We may want to revisit this as I'm not sure what the current best practice is.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[PiyushChandra17]

Nicely Done

[raclim]

Thanks for your work on this!