Make locations hideable

[smessie]

By having the possibility to hide / deactivate a location, locations that currently do not offer a place to study, but might in the future, can be hidden from the overview page, making it easier for students to find a location where they can study.

The location manager and application admin should be able to change this setting such that GSR can contact all locations leading up to the new study period, and hide all locations that are not answering for the time being.

Hidden locations should be excluded from the overview APIs (both default and StadGent one). Whether or not to make them visible when directly going to the location with the URL (with e.g. a warning) is to be decided.