Improve the accessibility of our jump-to functionality

Improve the accessibility of our jump-to functionality
- Stop hiding with display: none;, this hides our jump links from modern screen readers and users with motor disabilities (ie: nowadays, pratically everyone they are intended to help).
- Instead hide with an overflow that will make the links viable <tab> targets. This alone is enough to help screen reader users.
- Add in a script that will show the jump-links area on-focus for motor-impared users who can still see who have js enabled (this can't be done with css unfortunately)