Resolves Issue518 - Don't overwrite modal-inner when classname is passed

[amir-hossein-zarei]

Resolving #518 by customizing Modal className property inspired by React Modal buildCalssName

Author Checklist

• [x] Add unit test(s)
• [ ] Update version in package.json (see the versioning guidelines)
• [ ] Update documentation (if necessary)
• [ ] Add story to storybook (if necessary)
• [ ] Assign dev reviewer

[chawes13]

@Amihozar Thanks for the PR! If you use the word "resolve", "resolves", or "resolved" in the PR description, then the issue will be linked and automatically close when this PR is merged. However, "resolving" is not one of those key words. More info here: https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue

[chawes13]

This PR is deprecated by https://github.com/LaunchPadLab/lp-components/pull/554. Thanks for the initial contribution that inspired the final result!