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

Rename MWBS service to Merge When Pipeline Succeeds

## What does this MR do?

This MR renames Merge When Build Succeeds feature to Merge When Pipeline Succeeds, because we recently changed the behavior, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6675.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] All builds are passing

## What are the relevant issue numbers?

Closes #23354

See merge request !7135

Extend code review docs with chapter about the right balance

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See merge request !7838

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