Simplify JSX a11y rules to use the recommended ones

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• uses the jsx-a11y/recommended rules
• with one exception for label-has-for, which is deprecated in v6.1.0 in favor of label-has-associated-control but we still want to require only for/id and not nesting
• sets react/button-has-type to warning as this rule is pretty useless in its current state
• adds an exception for translators comments in capitalized-comments

Fixes #12

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Note: in a first moment I thought lowering to warnings a few rules was a good idea. Reverted that, as we're going to add exceptions where needed in the various projects. See also this previous change 596245d

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I've pushed two branches for the plugin, to test the new ESLint rules. This PR changes are not released yet so to test there's the need to yarn-link the local copy of this branch.

free: eslint-jsx-a11y-new-config
premium: eslint-config-premium

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Test branch for yoast-components: eslint-new-rules-adjustments
(0 errors, 356 warnings 😄)

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Test branch for YoastSEO.js: eslint-jsx-a11y-config