Harmonisation of experiments web API

We presently have our web API for experiments in a bit of an inconsistent state:

1. Some of the calls go through /experiments
2. Some others go through /experiments/configuration

This is not the case for other platform objects (such as data formats, toolchains, or else). This creates a slight complication for API users.

@philip.abbet: is this just legacy behaviour or is there a good reason for keeping it this way?