add APP_KEY generation and parameter

[jessebot]

Apparently this is necessary and I completely missed it because it's not in the main docs I was a reading:
https://github.com/pixelfed/pixelfed/blob/13af8c5133aef4708f599488f678c09325ac417c/.env.docker#L893-L904

This PR adds these parameters:

pixelfed:
  # app specific settings
  app:
    # -- This key is used by the Illuminate encrypter service and should
    # be set to a random, 32 character string, otherwise these encrypted strings
    # will not be safe. If you don't generate one, we'll generate one for you
    # however it will change everytime you upgrade the helm chart, so it should
    # only be used for testing. In production, please set this, or pixelfed.app.existingSecret
    key: ""
    # -- use an existing Kuberentes Secret to store the app key
    # If set, ignores pixelfed.app.key
    existingSecret: ""
    # -- key in pixelfed.app.existingSecret to use for the app key
    existingSecretKey: ""