Accessibility and keyboard navigation issues

[mackenly]

Keyboard navigation and accessibility in general have a few issues. Namely the sponsor's slider. Creating this issue to hopefully work on some fixes.

https://github.com/tricities-dev/tricities.dev/tree/feature/a11y-fixes

[mackenly]

I've made a few commit here but won't PR yet because it still needs quite a bit of work. https://github.com/tricities-dev/tricities.dev/tree/feature/a11y-fixes