Add a settings popover to exclude post-types from tracking

[aristath]

Context

This PR adds a popover for settings, to allow excluding post-types from tracking.

The settings popover was added to the header, right next to the months selector:

Initially, I wanted to add a setting to select which post-types will be included, however that would lead to lots of complications if post-types get added later. With that in mind, it seemed a better option to add a setting to exclude post-types.
To avoid designing a new page etc, it seemed better to reuse the popover API we already implement - especially since we only have 1 option. If in the future we add more options, we may want to rethink that approach

Summary

This PR can be summarized in the following changelog entry:

• Add settings popover to exclude post-types from tracking.

Relevant technical choices:

• Used an AJAX call to save the options, to avoid reloading the page.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

[github-actions]

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[jdevalk]

I've been discussing this with Marieke. We'd like to change it; instead of making it "Exclude", I think we should make it "Include" and we should default to Posts and Pages "on" and the rest "off".

[jdevalk]

Also; hitting "Save" should trigger a page refresh, as it changes the data that should be displayed.