add xxl breakpoint and xxl prop everywhere applicable

[nlambert]

feat(col, container, jumbotron, ul, p, dt, dd, form, navbar, modal, label): xxl breakpoint and prop

add xxl breakpoint and xxl prop everywhere applicable

v4

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[kopax]

Hello, I merged this too quickly as travis-ci.org is now down replaced by app.travis-ci.com.

I had to fix travis-ci.org for travis-ci.com and your test does not pass here : https://app.travis-ci.com/github/bootstrap-styled/v4/jobs/572502034, now dev branch is not passing the CI.

Mostly no-multiple-empty-lines errors.

Can you run eslint with --fix and resubmit a PR in order to publish the release through travis for you, ex:

/home/travis/build/bootstrap-styled/v4/src/Table/tests/Table.test.js
  26:1  error  More than 1 blank line not allowed  no-multiple-empty-lines
  33:1  error  More than 1 blank line not allowed  no-multiple-empty-lines
/home/travis/build/bootstrap-styled/v4/src/Table/tests/Td.test.js
  29:1  error  More than 1 blank line not allowed  no-multiple-empty-lines
/home/travis/build/bootstrap-styled/v4/src/Ul/index.js
  18:1  error  More than 1 blank line not allowed  no-multiple-empty-lines
/home/travis/build/bootstrap-styled/v4/src/Ul/tests/index.test.js
  11:1  error  More than 1 blank line not allowed  no-multiple-empty-lines
✖ 221 problems (221 errors, 0 warnings)
  221 errors and 0 warnings potentially fixable with the `--fix` option.
The command "npm run test" exited with 1.

Thanks for contributing.