fix: use correct classes for outline buttons

[smonolo]

Description

This PR introduces a series of fixes to outline buttons used around the website, which will occasionally have a strange behavior if btn-white class is not coupled with btn-outline-primary.

Checklist

• I followed the indications in CONTRIBUTING.md
• The documentation related to the proposed change has been updated accordingly (also comments in code).
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?
• Ready for review! 🚀

Fixes

Fixes #1457

[smonolo]

Ready for review.

It might be also worth considering to create a shared component (include) with the buttons that exposes a prop to pass the Slack channel link, and import them everywhere is needed, so they will always look the same (right now they are all a copy paste).

[bfabio]

Thanks @smonolo, looks like #1327 and #1093 didn't cover every button in the site.

A shared include could be an improvement. More structural solutions, ordered by increasing levels of effort are:

• Update Bootstrap Italia #1302
• Port to Next.js #1007