This changes ProjectsFinder#init_collection so it no longer relies on a
UNION. For example, to get starred projects of a user we used to run:

    SELECT projects.*
    FROM projects
    WHERE projects.pending_delete = 'f'
    AND (
        projects.id IN (
            SELECT projects.id
            FROM projects
            INNER JOIN users_star_projects
                ON users_star_projects.project_id = projects.id
            INNER JOIN project_authorizations
                ON projects.id = project_authorizations.project_id
            WHERE projects.pending_delete = 'f'
            AND project_authorizations.user_id = 1
            AND users_star_projects.user_id = 1

            UNION

            SELECT projects.id
            FROM projects
            INNER JOIN users_star_projects
                ON users_star_projects.project_id = projects.id
            WHERE projects.visibility_level IN (20, 10)
            AND users_star_projects.user_id = 1
        )
    )
    ORDER BY projects.id DESC;

With these changes the above query is turned into the following instead:

    SELECT projects.*
    FROM projects
    INNER JOIN users_star_projects
        ON users_star_projects.project_id = projects.id
    WHERE projects.pending_delete = 'f'
    AND (
        EXISTS (
            SELECT 1
            FROM project_authorizations
            WHERE project_authorizations.user_id = 1
            AND (project_id = projects.id)
        )
        OR projects.visibility_level IN (20,10)
    )
    AND users_star_projects.user_id = 1
    ORDER BY projects.id DESC;

This query in turn produces a better execution plan and takes less time,
though the difference is only a few milliseconds (this however depends
on the amount of data involved and additional conditions that may be
added).