Add admin capability to filter users by solutions submitted

[madebydna]

Specifically, we need filters for:

• submitted a correct solution
• submitted at least one solution
• submitted no solutions

[gjp]

Will this be met by meeting one of Jordan's requests on https://github.com/rmu/puzzlenode/issues/25

• UsersController#index should allow sorting by fields

since both Solved and Attempted will become sortable, or would you prefer this be added to the search form?