Generalise xpub-edit and make it easier to configure

New ConfigurableEditor component (better name suggestions?) which can be configured by passing props instead of messing with ProseMirror schemas.

Also update the existing editors to use this one, removing a lot of duplicate code. The only difference I can tell is that the dropCursor and gapCursor plugins are no longer enabled. From what I can tell it doesn't look like their functionality would have been used (one is about dragging things over the editor and the other is about positioning the cursor around blocks like tables and images).

Note, I have only tested this in the styleguide.