Rework tab-indexing and mnemonics

[krestenlaust]

Tab-indexing on the frontpage is inconsistent and it's difficult to navigate the page, either remove the custom tab-indexes, or redo them to make them intuitive. The same goes for the mnemonics (access keys), it would be nice to have some good shortcuts.

https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex
https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/accesskey

[atjn]

For tab indexes, I would just remove them. The autofocus attribute will ensure that "buy" is the first thing you can click and/or tab to, and as you say, if you want to go back to the buttons in the header, you will intuitively expect to tab backwards, but for some reason we have made it so you need to tab forwards.