fix: accessibility issues

[joostvanderborg]

Fixes #1058:

• The Logo link was not concrete on where it linked. Changes the screenreader text from 'Hoofdpagina' to 'Homepage Mijn Nijmegen'.
• Hide icons in header (logout / nijmegen.nl link) from accessibility tools.
  • Logout is not explicitly added to the title 'Uitloggen', since the only hint this was a logout link was the icon.
  • The other icon has no real content value, since the link text being 'nijmegen.nl' already fully explains where the link will lead.

Fixes #1059: Underline links in header, when hovered, to show the link is a link, and it's under the cursor.