Add test pages for colour schemes

[larsyencken]

What

Add two new test pages for colour schemes, as per @danyx23's idea:

• [ ] All the charts with non-default color schemes in either the main color scheme or the map color scheme
• [ ] All charts for a given color scheme

Why

Authors want to be able to review colour schemes in bulk, so that they can replace or remove bad ones as part of our cleanup project.

[marcelgerber]

This is something we can easily integrate into the "Bulk downloads" feature in the admin.

[danyx23]

Talked about this with Marcel - we talked about this in the wider context of https://github.com/owid/owid-issues/issues/262 . We think that two new pages should be added to the tests page. The two test pages should do the following:

• One shows all the charts with non-default color schemes in either the main color scheme or the map color scheme
• The other shows all charts for a given color scheme

This should allow Marwa and the authors to get a good feeling for currently used color schemes and think about which to remove or add/replaced

The bulk csv export should also be extended with the baseColorScheme from the chart and the map for easy searching by the data team.

[larsyencken]

Updated the title and scope now to reflect your suggestion

[larsyencken]

Marking it as "nice to have" until we get a clearer signal from Marwa that we still want this.

[danyx23]

Implemented this as as series of observable notebooks as part of the #1611 project (see this notion page for the links to the notebooks)