Refactor block settings

[ravewebdev]

Closes #91

DESCRIPTION

• Abstracts custom color palette control to new component, ColorPaletteControl.
• Abstracts background options panel to new component, BackgroundSettingsPanel.
• Abstracts BackgroundSettingsPanel's media controls to new component, MediaControl.
• Adds ContrastChecker component to Slide and Accordions blocks to check color contrast and output warning if needed.
• Updates Slide component to use new panel/control components.
• Updates Accordions block to use custom color HOCs in save function and withColors HOC in edit function (fixes color selection functionality).
• Removes custom color palette from global config.js.

SCREENSHOTS

Slide block showing color contrast warning when using background color:

Accordions block showing color selections:

OTHER

• Is this issue accessible? (Section 508/WCAG 2.0AA)
• Does this issue pass linting? (ESLint)

STEPS TO VERIFY

1. Add Carousel block.
2. Customize font color and background type, switching between background color, image, video. Confirm preview and frontend update accordingly.
3. Choose similar font and background colors to trigger contrast warning.
4. Add Accordion block.
5. Customized font and background colors. Confirm preview and frontend update accordingly. Confirm checkmarks show on currently selected color(s).
6. Choose similar font and background colors to trigger contrast warning.

DOCUMENTATION

Will this pull request require updating the WDS Blocks wiki?
Documentation included in README.md files for new components. Further documentation to be added as per #88.

[gregrickaby]

@ravewebdev 🥇