UI: update font family

[Hypnosphi]

Based on Slack discussion

[danielduan]

I think Helvetica Neue is still necessary for OS X 10.11 and below when not in Chrome:
https://bitsofco.de/the-new-system-font-stack/

[Hypnosphi]

Shouldn't Arial be available there?

[danielduan]

Yes it is. I think it's included to conform to the system's original styles and Arial is an unfamiliar and clunky looking font to Mac users.

[Hypnosphi]

Well if the alternative is to have a font that jumps when going from normal to bold (and we have a menu in which it happens all the time), the unfamiliarity is a lesser evil

[danielduan]

hm fair enough.

[codecov]

Codecov Report

Merging #1560 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1560      +/-   ##
==========================================
+ Coverage   20.43%   20.46%   +0.02%     
==========================================
  Files         241      241              
  Lines        5250     5243       -7     
  Branches      649      647       -2     
==========================================
  Hits         1073     1073              
  Misses       3682     3682              
+ Partials      495      488       -7

Impacted Files	Coverage Δ
lib/ui/src/modules/ui/components/theme.js	100% <ø> (ø)	⬆️
lib/ui/src/libs/key_events.js	23.25% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
addons/knobs/src/react/WrapStory.js	12% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/down_panel.js	23.52% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
lib/ui/src/modules/shortcuts/actions/shortcuts.js	6.25% <0%> (ø)	⬆️
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
... and 14 more

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[igor-dv]

When I mentioned changing the fonts is inconstant in the prev PR, I meant that IMO it's wrong to change this only in one place (lib/ui). This kind of theme is used in a few other places around this repo (for example addons), and styleguide-wise I think all the fonts should be the same (at least in the official addons / UI). But maybe I 'm too picky =) Anyway if it solves this jumping glitch - 👍 👍

[Hypnosphi]

Well, in fact there isn't any consistensy of font-family across the codebase at the moment: https://github.com/storybooks/storybook/search?utf8=%E2%9C%93&q=fontFamily&type=

And if we want to make it more consistent, it would be better to use a shared theme instead of repeating ourselves. This can be a part of the major task of moving UI components to lib/components