Finalises experience page (for now...)

* Adds a nice "summary" at the top, using the same blue cover as on the
home page
* Adds an interactive timeline. Tries to abstract the JS, finding
similarities in the modal functionality
* Adds a further skills section. I might clean this up later, but for
now seems OK
Adds more saturation to site colours
Replaces `background` with `background-color`
Updates ::selection style

* Now blue on black
* Removes special styling for code ::selection
Commits missing experience background image