Remove old styles for IE7 and below

Those browsers are old, forgotten, and aren't used anymore anyway. And the
website probably doesn't work in those old browsers anyway.
Improve layout for /planning (both on mobile and desktop)

On bigger screens the full width wasn't used, and on smaller screens the whole
page would scroll if the table didn't fit.
Fix: the scrollbar was always visible (oops!)
Merge pull request #204 from aykevl93/planning-layout

Planning layout
header: update bestuul
add headers.README
Merge pull request #200 from aykevl93/drop-ie7-styles

Verwijder IE7-specifieke CSS
Use `utf-8` instead of `utf8`. This fixes a problem with IE9

`utf8` is non-standard, but is accepted by most other browsers. `utf-8` it shall
be.
Merge pull request #205 from aykevl93/fix-invalid-charset

Use `utf-8` instead of `utf8`. This fixes a problem with IE9