Fix ESLint and Stylelint violations

[jjroelofs]

Linked issues

• Fix ESLint and Stylelint violations #800

Solution

• Stylelint: Shortened 6 hex colors in scss/base/variables.scss (#222222 → #222, etc.)
• ESLint: Auto-fixed 33 Prettier formatting violations in Gruntfile.js, postcss.config.js, webpack.config.js (single → double quotes, indentation). Added inline disables for 3 unavoidable config file patterns (import/extensions, global-require, import/no-extraneous-dependencies).

Checklist

• I have read the CONTRIBUTING.md document.
• My commit messages follow the contributing standards and style of this project.
• My code follows the coding standards and style of this project.
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Need to run update.php after code changes
• Requires a change to end-user documentation.
• Requires a change to developer documentation.
• Requires a change to QA tests.
• Requires a new QA test.
• I have updated the documentation accordingly.
• All new and existing tests passed.