added more test coverage for contractor rates feature

* fixed contracts controller test - added coverage for specifying a contractor rate
* added coverage for contracts model: billable amount total, user project rate or default, and members with entries
* formatted rate in form entry as 0.00
* when addeding a new contract, user's without a project rate have a default rate set of 0.00
* when updating an existing contract, user's without a project rate get auto-populated to the contract's "global hourly rate"