Boost: Add "load default strings" to excludes in concatenate UI

[dilirity]

Proposed changes:

• Add a button, similar to cornerstone pages, that loads default exclude values for concatenate JS and CSS;
• Clean up CSS in minify component.

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

n/a

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

no

Testing instructions:

• Setup Boost;
• Make sure concatenate JS/CSS UI is behaving as expected.

[github-actions]

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[donnchawp]

Some minor things, but looks good otherwise.

[haqadn]

Looks good to me. I did make some changes. Since I am the one who made the latest changes, leaving the approval to others.

Rationale behind my commit (b2c5854):
I wanted to keep the stores for CSS and JS separate. This will allow us to easily move the store registrations to individual modules, which we need to do soon.

[dilirity]

Changes @haqadn did look good.

I think we can

I can't approve, since it's my pr 😅

[donnchawp]

Looks good!