Proposal: Add ProjectContract FK to Entry model

• Add ProjectContract FK to Entry model (null=False)
  • Write data migration to associate FK to the project contract (of the entries' project) based on the date of entry. For most cases, project contracts have not overlapped.
• Update the invoice page to list contracts, not projects.
• Update entry create/edit page to:
  • Include a contract drop down (only show associated project contracts).
  • When a project is selected, refresh the contract list to only show associated contracts (via AJAX).
  • By default, auto-select the first contract that appears in the list.