Add aria-label's to buttons/links without text

[felixwetell]

Buttons and links without a text should contain a aria-label to let screen readers know what that element does.

[felixwetell]

Landing page

• [x] Sponsor images on first page should have aria-labels

Footer

• [x] KAU and HHK images
• [x] Social media

Navbar

• [x] Navbar brand

Partners

• [x] Partner image

Associations

• [x] Association image