Components: Extract Reusable Post Sticky Component

[youknowriad]

Related to #2761 (comment)

All these (the sidebar panels) must be refactored slightly to drop all the "layout" pieces and keep only the "forms". For example, the sidebar panels like PostTaxonomies, we should extract only the PostTaxonomyForms into a reusable component without the expandable panel behavior that is specific to our current UI. (This step could be done in a separate PR preceding the directory structure change)

This is the first one of multiple PRs addressing the comment above and extracting reusable pieces from our current sidebar panels. This PR does so for the PostSticky component.

Testing instructions

• Test that the post sticky toggle still works and shows as expected.

[youknowriad]

To avoid duplicating the "support sticky" check, I extracted it to its own component. I could reuse this approach for all the other sidebar panels.

[mcsf]

This works well.

Otherwise, I could imagine the following:

// new HoC
const withUserPrivileges = ( predicate ) =>
  flowRight(
    withAPIData( constant( { user: '/wp/v2…' } ) ),
    withRenderIf( predicate )
  );

// consumer
import withUserPrivilege from …;
const applyPostStickyCheck = withUserPrivileges( ( { postType, user } ) =>
  postType !== 'post' ||
    ! user.data ||
    ! user.data.capabilities.publish_posts ||
    ! user.data.capabilities.edit_others_posts
);

export default applyPostStickyCheck( MyComponent );

[mcsf]

This could obfuscate component render logic, though. Right now most HoCs only deal with prop injection and side effects (like click-outside), not affecting when things should be rendered. Which is why the current way seems appropriate. 👌

[codecov]

Codecov Report

Merging #3114 into master will decrease coverage by 0.06%.
The diff coverage is 26.31%.

@@            Coverage Diff             @@
##           master    #3114      +/-   ##
==========================================
- Coverage   32.23%   32.17%   -0.07%     
==========================================
  Files         216      218       +2     
  Lines        6151     6160       +9     
  Branches     1081     1083       +2     
==========================================
- Hits         1983     1982       -1     
- Misses       3517     3525       +8     
- Partials      651      653       +2

Impacted Files	Coverage Δ
editor/sidebar/post-sticky/index.js	0% <0%> (-54.55%)	⬇️
editor/post-sticky/index.js	0% <0%> (ø)
editor/post-sticky/check.js	71.42% <71.42%> (ø)

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[mcsf]

Nice and clean.