Add Contract Fields to Job Offer + Defaults for Job Offer + Add flex delft export on all applications page + Change export declarations to aggregate

Description

• Adds "FlexDelft" Export Button - With columns: Start Contract, End Contract, Baan-Code, Student Name, Payscale, Remarks
• Declarations are aggregated by Baan-Code and Student for Simplicity
• Adds contract fields to JobOffer entity
• Adds coordinator defaults to JobOffer entity
  • The coordinator defaults for ExtraWork haven't been removed since they're already implemented, and perhaps a different faculty would need them.

Related Issues

• Resolves #99 (closed)
• Resolves #101 (closed)

Checklist

• I have added a changelog entry to reflect the significant changes I made.