Issue78 list secondary services

[JohanKJSchreurs]

Initial implementation of Secondary Services in the aggregator

This implements issue #78

First implement the following functionality:

• Listing service types
• List services for the current/logged in user.
• Create, update and delete a new service.

[soxofaan]

Already looks good!

did a first pass of some review notes

[soxofaan]

Not sure yet, but I'm wondering if adding these auth related constants is indication that the tests that use these better belong under test_views.

[JohanKJSchreurs]

Good point.

In fact, the corresponding parts of test_views.py and test_backend.py are already testing very similar scenarios from two different points of view, one time directly on the AggregatorSecondaryServices and then again in the views using HTTP requests. Maybe that is useful, but it could just as well be too much duplication.

The main difficulty that I see here to be able to avoid extra auth setup in test_backend.py is that in the AggregatorSecondaryServices we are basically proxying the other services via BackendConnections. We are not directly using other instances of SecondaryServices, in which case we could simply pass the user_id argument to the other instance.
As a result I had to use the Flask request to pass on the authentication info to the backend connection.

Maybe there is a better way to do that, but with this method of passing authentication to the backend connection I haven't found a better way to set up authentication in the tests. Essentially it needs a request context so that authentication headers can be passed to the connection.

[soxofaan]

Merged this PR manually (after rebase) in 39d0b89.
great work, thanks.

Follow-up issues and discussion will go to #78