Boost: make Concatenate modules work more consistently

[donnchawp]

The CSS and JS Concatenate modules do not work in the same way other modules on the settings page work. This PR update them to fix that.
Instead of a cancel button, it uses the same show hide link other modules use, with up/down chevrons. I think I'd rather leave the text of the link, instead of changing it to "Hide" as the cursor has to be moved if it was not in the right place.
If the text input form is closed, the value of the form is reset to the saved value now. It would retain an edited version before, unlike other modules.

Fixes #36274

Proposed changes:

• Wrap the SCSS in a wrapper selector and adjust CSS to work.
• Use an up/down chevron to show and hide the meta form.
• Show/hide link uses isExpanded state like other modules now.
• Remove the Cancel button
• Add a description and help text.
• Remove inputLabel parameter from MetaComponent as it uses datasyncKey to decide if JS or CSS.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to settings page and open and close the two modules
• Edit the settings and save them.
• Reload the page to confirm the changes stick.

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