Accessibility - Tab index improvements for the default banner markup

It's been reported that the default banner design can be improved to be more accessible to screen readers, by fixing the following issues:

Consent categories checkboxes are still tabbable while hidden Currently we set the height of that section to 0 instead of hiding it altogether, so screen readers still pick it up and announce it. ref https://github.com/humanmade/altis-consent/blob/main/assets/sass/banner/_transitions.scss#L14
Consent banner should be filterable to be the first tabbable item in the page (?) Some implementations might prefer the cookie consent to be the first tabbable item in the page, so customers would decide on their preference before dealing with the page. While that's not a regulation requirement, it'd be convenient for implementations to use a filter to make that happen.

[ ] Tabbing through page links should not navigate to hidden checkboxes of the consent banner if they're not visible. Changing height: 0 to display: none and carrying out necessary changes to make that work.
[ ] Altis has a filter altis.consent.consent_banner_tabindex, defaulting to 0, on the consent banner container.

humanmade / altis-consent