Followup work from moving renku notebooks

[olevski]

Leftover from renku notebooks code move:

Tasks

• notebooks: convert http calls to direct calls #396
• Implement additional API input validation #398
• Move operational logic from blueprints to core #399
• Evaluate trust-manager to replace renku/certificates #400
• components/renku_data_services/notebooks/api/notebooks.py L677: the error message could be more descriptive, if I saw this in a log, I wouldn't know what the cause was.
• components/renku_data_services/notebooks/api/notebooks.py L683 and 685: isn't this what empty is for?
• components/renku_data_services/notebooks/config/init.py: TODO for later, move out of init.py
• components/renku_data_services/notebooks/util/authn.py: We need to refactor this code. we create a RegisteredUser no matter what, check if authenticated is True (which it is if all required headers are present, but we don't actually check the content of the headers in the RegisteredUser init) and if not create an AnonymousUser. This seems pretty brittle and is a weird way for this code to be written. and if I read it correctly, I could put whatever I want in those headers and it would consider it a registered user?
• Rename NotebooksNewBP to SessionsBP or something similar.
• Improve makefile and dev workflow for new sessions

Related to #301