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[ci skip] UX guide: clean up term tables in copy

[ci skip] UX guide: update dropdown max height for multiline items

[ci skip] UX Guide: Add retry to blue

[ci skip] UX Guide: refine discussion definition

Add guidance on timings and easing

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Detail when not to use easing

Add dropdown and hover examples

Add quick update animation

Describe hover, focus and active states for primary and secondary
buttons.

Organized the icons section in a table. Replaced images for icons
so they are all the same size.

Organized the color section in a table. Replaced the images with
new ones so the table would have a more manageable size. Updated
the values for some colors to show the $color-light shade.

Remove commented sections

Remove `db/schema.rb`

Add db/schema.rb from origin/master

Fix typos