Fixing input validation messaging.

[parkeryoung]

In bootstrap 4 the classes for the input validations have changed. The
form-group div is now expected to have the has-danger class. The
message text is now supposed to have a form-control-feedback class
as opposed to an invalid-feedback class.

I removed the icon option as it doesn't seem to be supported by bootstrap anymore.

There are some parts I'm uncertain of, namely the places in the tests where it doesn't seem like the form-control-feedback is rendered. I'm not sure if we are supposed to put the has-danger class on the div in these instances.

This also includes an update to the README to reflect this change.

If there is anything that needs to be changed, let me know.

[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):

* [#442](https://github.com/bootstrap-ruby/bootstrap_form/pull/442): Fixing input validation messaging. - [@parkeryoung](https://github.com/parkeryoung).

Generated by 🚫 Danger

[lcreid]

Thanks for the contribution!

has-danger was in the alpha release of Bootstrap 4. According to our PR #370, Bootstrap 4 Beta 3 got rid of has-danger for a different way of marking validation errors. The second-last bullet here also seems to indicate that has-danger no longer exists.

There is still some work in progress on validation error handling for check boxes and radio buttons, but I believe the validation errors other types of inputs are working in the master branch now. (Admittedly, many of those changes were merged into master recently.) Can you please explain further why you believe this PR is necessary?

Again, thanks again for the contribution.

[parkeryoung]

@lcreid Oh my bad, I was looking at the wrong documentation and I suppose my project is behind the times. I'll close this, thanks for the feedback. I'll update my application's bootstrap version, If I notice anything else I'll be sure to send another PR, hopefully then it might actually be helpful.

[lcreid]

No worries. There were so many changes through the different betas of Bootstrap 4 that we've all experienced what you did. If you're interested in contributing, check the issues list. If an issue has an obvious answer and no one is working on it, add a comment briefly describing how you plan to solve it, then have a go. There aren't any issues like that on the list right now, but I suspect we'll have a few in a week or so.

Thanks again for your willingness to help!