Improve performance for user database queries

[raclim]

Changes:

• adds the .lean() option to user MongoDB queries to potentially improve performance
• indexes the user property on the project schema

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

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