Fixes focus indicators

[SaptakS]

Refs #1381

Improves the focus indicators. Based on WCAG 2.2 standards, it is a better idea to have solid outlines instead of a dotted outline for focus indicators. Also, color change for focus indication isn't great because it doesn't often work for color blinded people.

So things done in this PR:

• Make all anchor tags have a solid 2px outline of the currentColor (Sadly the currentColor itself doesn't meet WCAG color contrast requirement, so the focus indicator doesn't either, but hopefully that's a separate discussion)
• Apply the same above design to all buttons as well
• Adds an offsetted focus outline for the CTAs
• Moves padding in navigation elements from the <a> element to the <li> element.

These address the below concerns by @thibaudcolas in #1381

Focus style almost invisible on "Get started with Django" CTA

Custom "dotted outline" focus styles: not visible enough and broken in main navigation

Attaching some screenshots of the changes:

[SaptakS]

Thanks @bmispelon for pointing those out. Have updated them.

[bmispelon]

Thanks for the quick update. This looks good to me now, unless anyone objects I'll merge and deploy this over the coming weekend.

[bmispelon]

And it's live, well done everybody 👏🏻