Search multiple corpora

A much requested feature is the ability to search multiple corpora simultaneously.

It should be relatively modest task to allow users to search any selection of corpora with _limited_ functionality, but some functions are a lot more involved. Many features such as filtering and visualisations become troublesome to interpret when corpora don't share the same fields. A potential solution is to implement 'core fields' (#993).

Another solution would be to focus on cross-corpus search in specific subsets that have similar definitions, such as newspaper corpora. This is less flexible to the user but would probably satisfy the most common use cases.