Implement changes from 2020 salary negotiations

This is a scheduled issue to implement changes arising from the 2020 pay negotiations.

Please note, whoever picks this issue up should remove the other assignees and then raise the following issues:

• an issue in the webapp to deploy the resulting changes
• an issue in the salaries-commitments to deploy the resulting changes
• a copy of this issue for 2021 with a due date set to 2021-09-01 (also assign to members of the team so that they are notified when due)

For good measure, an event matching this issue has been added to the shared Google Calendar. This event repeats so no additional action is necessary.