Redesign dashboard

[ml-evs]

There were quite a few comments at the workshop that the current design of the dashboard should be reconsidered, given that the use of the dashboard has evolved from just validating providers to also being the primary user-facing list.

We should just make the list of providers have collapsible sections for the lists of child databases, rather than a new page per provider. This could be achieved using simple HTML or something like htmx, without needing to write any JS.