Improve usability of header on mobile devices

Closes #762 (closed)

Previous behaviour

Steps to reproduce:

1. Use the site on a small mobile device
2. Header sticks to the top
3. Breaking of header buttons to next line

New behaviour

Steps to validate that it works:

1. Use the site on a small mobile device
2. Header now doesn't stick to the top, only on desktop
3. Breaking of header buttons to next line is fixed