Sort open job offers by cohort

Description

• Open job offers are now sorted by cohort (in reverse alphabetical order), then by start date (earliest first), then by name (alphabetical order)
  • This means that CSE2020 is before CSE2019 which is in turn before CE2020

Related Issues

• Resolves #82 (closed)

Checklist

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