Color Palette: Fix the active color using the value prop

[youknowriad]

This fixes the first part of #2324

Also fixes some React Warnings when the color was unset.

[codecov]

Codecov Report

Merging #2361 into master will decrease coverage by 0.74%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2361      +/-   ##
==========================================
- Coverage   25.06%   24.32%   -0.75%     
==========================================
  Files         153      155       +2     
  Lines        4760     4905     +145     
  Branches      801      829      +28     
==========================================
  Hits         1193     1193              
- Misses       3014     3131     +117     
- Partials      553      581      +28

Impacted Files	Coverage Δ
blocks/library/cover-text/index.js	35% <ø> (ø)	⬆️
blocks/library/button/index.js	26.66% <ø> (ø)	⬆️
editor/sidebar/post-taxonomies/index.js	0% <0%> (ø)	⬆️
...itor/sidebar/post-taxonomies/flat-term-selector.js	0% <0%> (ø)
...ebar/post-taxonomies/hierarchical-term-selector.js	0% <0%> (ø)

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[aduth]

When creating a Cover Text block, adding text, selecting colors, and refreshing, the block shows as externally modified.

[youknowriad]

weird @codecov this doesn't affect the coverage in theory (probably related to #2354)

[aduth]

Oh, I see the issue, though dunno if we want to address here. This is causing the cover block to be saved with <p class="false"> in some cases?

gutenberg/blocks/library/cover-text/index.js

Line 134 in a0714e2

const className = dropCap && 'has-drop-cap';

[youknowriad]

@aduth Yeah, unrelated to this PR, I can fix it separately