Fix the ProxyFix after the move to Bullseye

With the move from Werkzeug 0.14 to 1.0 as part of the switch to Bullseye the sytax to enable the ProxyFix middleware has changed and the app was raising an exception:

Traceback (most recent call last):
  File "/app/manage.py", line 26, in <module>
    from webhook import app, db
  File "/app/webhook.py", line 58, in <module>
    from werkzeug.contrib.fixers import ProxyFix
ModuleNotFoundError: No module named 'werkzeug.contrib'

Use the new syntax as already documented in a comment.