Rethink button styling and fix a11y issues

[mauteri]

Currently, buttons used for RSVP and the buttons in the RSVP modal rely on styling from the core Button block (uses the classes components-button-group, wp-block-buttons, and wp-block-button).

In this ticket, I'd like us to remove the usage of those classes and use our own styles that work with majority of themes (and using the primary colors set by the theme).

Also, currently we are using anchor tags for these buttons. This decision was made to prevent issues with odd button styling in themes that I was noticing when using the button block classes. Since we are moving away from those button block classes, let's use the button element instead of the a element, so markup is semantic and is no longer an a11y no-no.

Relevant links to read:
https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/button_role (aria with anchor tags)
https://equalizedigital.com/events/ (a11y experts)