Change the availability facet

[psaiz]

Currently there is a facet for the on-demand entries.

Those records have a message like:

See https://opendata-qa.cern.ch/record/13066 as an example

The idea behind is very similar to what should happen with the entries that are in cold-storage, where the users have to request that some datasets should be made available.

As a first step, let's change the facet. Instead of a toggle, let's put a menu that looks for the availability field
in the dataset (see #117)