Release version 2.7.0

Features:
* Account deletion: Users can now delete their own accounts. Admins can
  delete any user account. Account deletion removes sessions and their
  contents created by the user and anonymizes data created through
  participation in other sessions.
* Auto-deletion: Accounts can be deleted automatically after a
  configurable period of inactivity.
* OpenID Connect: OIDC is now supported for authentication.
  Configuration discovery support is required.

Improvements:
* Public Session Pool: Added API endpoint to clone a session from the
  pool. Previously, cloning had to be performed by the client.

Bug fixes:
* Import/Export: The handling of session features during import has been
  fixed. The raw exported data can now be imported without further
  manipulation by the client.