Active, not started contests not exposed in the API

[nickygerritsen]

Since e2eaca5 we don't expose contests that are active but not started yet anymore in the API.

We did this because for some entities we don't want to expose them before contest start, but for the contest itself we do.

We had some discussion and proposals in Slack but no resolution yet, so I'm creating an issue so we can track it
.

[nickygerritsen]

Note that this has the consequence that the CCS autologin LightDM theme thingy will not work.