refs #498, #606 - Project contract enhancements

- Migrations to make Project <-> ProjectContract a many-to-many relationship, and add a name field to ProjectContract. These fields were both populated using the current single project.
- Removed unused methods from the ProjectContract and ContractAssignment models.
- Removed the ContractMilestone model.
- Solved #606 with <a href="https://github.com/caktus/django-timepiece/pull/609/files#L8R27">this change</a>.

See #498.