[new-ui] Panels & themes

[walaura]

Thinking out loud and writing docs about how to do themability here, comments welcome!

I also added new components for flex layouts (super exciting) and pink titles (actually exciting)

And made it so storybook automatically generates jest snaps, this seems pretty sensible because they are basically the same thing.

[codecov]

Codecov Report

Merging #1157 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1157   +/-   ##
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  Coverage   48.68%   48.68%           
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  Files          64       64           
  Lines        3003     3003           
  Branches      650      650           
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  Hits         1462     1462           
  Misses       1306     1306           
  Partials      235      235

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[acao]

@walaura thanks a bunch, this is looking so splendid! giving me all kinds of ideas for the frontbackend side of things, seeing things come to fruition on this side

[walaura]

yas!!! so excited to hear

[acao]

this makes perfect sense for now, but as you may know we are converting to typescript within a month ish by very popular demand. once we get some architectural changes implemented

[walaura]

omg honestly can't wait!! i feel a bit like freefalling in here not having typechecking. Can I use TS on this part already?

[acao]

looks good to merge! whatchu think @walaura