Handle inline radio buttons and check boxes for Bootstrap 4.

[lcreid]

The Bootstrap 4 tags for radio buttons and check boxes is more regular, so the helpers are somewhat shorter, at the cost of perhaps making the review more difficult.

If there's a way to show the diff without comparing white space, it might give you a better idea of the real changes, most notably in the test cases.

This PR partially addresses #395.

[bootstrap-ruby-bot]

	1 Warning
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#410](https://github.com/bootstrap-ruby/bootstrap_form/pull/410): Handle inline radio buttons and check boxes for Bootstrap 4. - [@lcreid](https://github.com/lcreid).

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[mattbrictson]

FYI you can append ?w=1 to a PR diff to ignore white space, like this: https://github.com/bootstrap-ruby/bootstrap_form/pull/410/files?w=1

[mattbrictson]

Looks great. Nice code cleanup ✨

I do however see an issue that maybe warrants a separate issue or PR. Our documentation is not clear on how you properly create an inline form.

Some parts of the README imply that you just add layout: :inline to bootstrap_form_for and you are done. But that is either broken or the doc is wrong, because as I see in the tests, you must explicitly pass inline: true to each radio or checkbox (in addition to passing layout: :inline for the form?).

Furthermore the inline example in demo/ makes the same mistake, and the inline checkboxes there do not render nicely as a result.

I think we need to either change both the demo and the README, or our radio/check helpers need to perhaps assume inline: true if they are within a form that is layout: :inline. The latter sounds like it could be harder to get right.

[lcreid]

Nice catch. I was so into just mechanically changing the existing test cases that I didn't notice that we weren't testing the case where the form has the layout: inline. In the spirit of small, bite-sized PRs, I would humbly suggest we merge this one, and raise a new issue for the form-level inline. I won't be putting in an issue tonight, but I can do it in the morning if you haven't already done so.

[mattbrictson]

Sounds good! If you could open the issue tomorrow I'd appreciate it.