Add MAT feature to enable users to query based on multiple domains

During the October 2022 OONI Community Meeting, participants mentioned that it would be useful to be able to use the MAT to plot charts based on multiple domains (instead of just one). Specifically, this is useful for comparing the testing of multiple selected domains.

This is currently (unofficially) supported, as you can do this by selecting domain in the Y axis and then adding all the domains you want in the Search field of the table. However, it's not clear at all that users can do this.

We should therefore add support to MAT for plotting charts based on multiple domains. Ideally, this would be one of the MAT buttons at the start of the page, so that users can easily discover this feature (and not think that they can only search based on one domain).