Register Font Awesome stylesheet once and print only when needed

[westonruter]

Summary of change:
The Font Awesome stylesheet is large and if it is not needed on the page then it wastes bandwidth and browser CPU cycles. It also contributes negatively to the overall CSS budget of 50KB on AMP pages (see ampproject/amp-wp#4095). Therefore, the stylesheet should only be printed when it is actually needed by a block.

This has the added benefit of preventing the Font Awesome stylesheet from blocking the rendering content that appears before the first block that makes use of it.

This PR also centralizes the registration of the Font Awesome stylesheet in the wp_default_styles action, eliminating duplicated references to the stylesheet path.

Have the changes in this PR been added to the documentation for this project?
Does not apply

How to test:

1. Activate Atomic Blocks.
2. Add a block that does not require Font Awesome.
3. Check the frontend to verify that the Font Awesome has not been printed to the page.
4. Add a block that does require Font Awesome (e.g. Social Links).
5. Check the frontend to verify that the Font Awesome stylesheet has been printed once right before the dependent block.

Fixes: #127

Suggested Changelog Entry:

• Improve frontend performance by only printing Font Awesome stylesheet when required.

[nickcernis]

Confirmed that:

• Font Awesome CSS does not load if no icon blocks present.
• CSS loads if a block such as AB Sharing is present.
• link element only appears once for multiple AB Sharing blocks.
• Blocks that list posts containing AB Sharing blocks, such as the Post Grid, don't appear to be negatively affected.

[mindctrl]

Thank you @westonruter! Works great.

Thanks for testing @nickcernis!

[westonruter]

FYI: Gutenberg core is now moving in this direction. See WordPress/gutenberg#21838 and its PR WordPress/gutenberg#22754.