Rename Experiment -> Block

[BeritJanssen]

What's now known as an Experiment will be renamed as the model Procedure Block.

Steps:

Backend

• Backend: Rename Experiment to Block #1147

Frontend

• Frontend: Rename Experiment component, type, hook & API urls to Block #1145

URLS & Slugs

• Update URLS & Slugs in both Backend & Frontend in one PR (see also below) #1180

NB: as mentioned by #1058, we might consider making the block route a subpath of the experiment/ endpoint, resulting in routing as follows:
experiment/{slug}/block/{id}

That would have the advantage that it is always clear to the user in which experiment they are currently following a block. In terms of ordering, this would mean we would have to rename the experiment/collection endpoint prior to renaming the ExperimentCollection model itself.