Refactor EmployeeList for clarity and performance

[crawfoal]

See if we can make it more clear what we're doing - the goal is simply to get a list of all employees at a particular gym, where each user is listed once, and all of their roles at that gym are listed.

Ideally this refactor would also improve performance, and we'd be able to remove the following bullet n+1 query whitelists:

• User => :roles
• User => :manager_story
• User => :setter_story