Add missing block border variables

[Scotchester]

A few variables corresponding to an update to cf-layout were never propagated to the overrides file.

Testing

1. Pull changes.
2. Run npm link from project root directory.
3. Generate a new CF project.
4. Confirm presence of new variables in cf-theme-overrides.less.

   Review

   • @contolini
   • @jimmynotjim
   • @ascott1

     Checklist

   • [ ] Changes are limited to a single goal (no scope creep)
   • [ ] Code can be automatically merged (no conflicts)
   • [ ] Code follows the standards laid out in the front end playbook
   • [ ] Passes all existing automated tests
   • [ ] New functions include new tests
   • [ ] New functions are documented (with a description, list of inputs, and expected output)
   • [ ] Placeholder code is flagged
   • [ ] Visually tested in supported browsers and devices
   • [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[jimmynotjim]

Woah, totally fell off this one :+1:

[Scotchester]

Thanks!