Create list_observations API endpoint

[hellais]

Similar to list_measurements in API, but that operators on observations

[hellais]

This has been implemented in the following commit: d64463f.

It's fully compatible with the interface of list_measurements.

To land it into master I would like to first figure out a nice plan for deploying this. This is connected to coming up with a plan for making the deployment of the API/backend smoother in the future.

[hellais]

I think we can consider this issue done, pending on the deployment topic.