Create interface for editing and managing (static) dashboard parameters

[nielsdejong]

There is currently no (easy) way to view and/or edit all the parameters present in a dashboard. Users may to add any number of values which are then available to all queries as a parameter, without having an explicit parameter selector for them.

These "static" parameters could be good for whitelabeling, data-access-URLs or credentials.

We probably want a simple UI here that lets users create /edit/delete a list of key/value pairs. We can take inspiration from the UI used for e.g. the rule-based styling interface, although much more simplified.