perf: remove CodeMirror/CSSLint to reduce bundle size & speed

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Removes some heavy front-end libraries (CodeMirror and CSSLint) to reduce webpack bundle size and build time, to hopefully avoid timeouts when running build jobs on CI.

BREAKING CHANGE: makes the UI for editing custom CSS a little plainer and less user-friendly, and removes lint-as-you-type features for this UI.

How to test the changes in this Pull Request:

1. Create or edit a newsletter post
2. Go to Newsletter Styles in the "More" menu:

3. Expand the "Custom CSS" panel and confirm there's a plain-text input here.
4. Enter some valid custom CSS and confirm that the editor preview updates with the styles in real-time:

5. Save, refresh, and confirm that the styles are saved and reloaded correctly (you can also check the custom_css meta value on the post).

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[adekbadek]

Thank you!