Basically overhaul the entire thing.
* Header will be a bit taller, at least before it becomes sticky, to include tagline and user avatars.
* Need to support avatars from various sources: SocialProfile T258994, future specific implementation to use uploads/existing onwiki files with attribution, maybe pull from github or phabricator lololol, definitely some sort of default that results in some people winding up with one specific solid colour and generally just plonks 1-3 letters on a background or something?
* Add jump-to-bottom button
* Upon scrolling down, change header height and incorporate extra features (jump-to-top, nav when appropriate T165921, a quick editing menu T131998)
* Possibly also add a config/user setting to just disable the header stickiness? (T173240)
* Make colour bar optional/have configurable colours in site config (see hassomecolours) T132073, and also just generally suck less in terms of html output
* Probably hide colour bar colours or greatly simplify them on fixed version - T161472