Improve supports for dashboards filters

Customisable Dashboard has embedded support for date filters and anonymous users. We could improve the API so that filters can be defined by the user of the dashboard, rather than having to rely on the default ones, as those might not apply to all datasources.

This means removing filters from Customisable Dashboards and add config option to toggle/override filters externally. We can then move current filters handling to the analytics dashboard implementation. This should ideally give us the chance to improve filters handling in general, and being able to have filters specific to datasources, which is something we discussed already.

Being date filters probably quite a popular one, we should make it so that it's easier to create a date filter and override change its options. I've also found an quirk where when set to Today, the end date and start date is set to the same data ( i.e. today ), and that does not work for o11y ( delta = 0 ). So we might want to take a look at some of those quirks as well.