Gutenberg - reimplementation of block borders

[dratwas]

Fixes wordpress-mobile/gutenberg-mobile#1948
Gutenberg-mobile PR - wordpress-mobile/gutenberg-mobile#2050
To test:

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading it from AppCenter here with build number: 25457. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

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