Selections for @context field

[sebastiankb]

When I create a new TD/TM by the assistance it would be good to get the option to select a number of context namespaces that may be integrated in the @context file. There can be a pre-defined list. If a context is not available manually input fields for prefix and namespace value can be provided.

We can start with a simple context list like schema.org, https://w3id.org/saref#, and http://www.ontology-of-units-of-measure.org/resource/om-2/