Add keywords

In our initial planning, we wanted to have keywords for configurations and projects/campaigns.

Part of this issue is to rethink where we can re-introduce the keywords.

Main point of keyword usage:

• sensorML
• support the search function

As we wanted to have the configuration to be mostly a measurement configuration, maybe it makes more sense to have keywords on devices and platforms.

In sensorML they are related with the describeSensor operation, so they should be more on the devices - in my opinion.

CC: @mhanisch