Barchart visualisations rely on search filter metadata

At the moment, if you use the the `resultscount` and `termfrequency` visualisation on a field, the field *must* have a search filter, or the visualisation will raise an error. However, this is not required from the user perspective.

Date fields must also define a `lower` and `upper` bound for the filter in order to use the visualisation, though this is optional for the filter itself. Corpora created through the corpus form don't specify the bounds, but should still be able to use the visualisation.

Change the logic for the frontend visualisation so it does not require filter metadata.