suggestion: use Fresh's active link styling

[iuioiua]

Active link styling logic can now be simplified thanks to Fresh 1.5.0's new functionality. To see how this can be achieved, see https://deno.com/blog/fresh-1.5#easier-active-link-styling

Here are some examples of where things can be improved:

• https://github.com/denoland/saaskit/blob/f23c69e6f3a75541fc2b303040d61c63e5334832/components/Footer.tsx#L75-L77
• https://github.com/denoland/saaskit/blob/f23c69e6f3a75541fc2b303040d61c63e5334832/components/Header.tsx#L75-L99

[iuioiua]

Completed in #632