Fix LICENSE for dashHtmlComponents installation

[rpkyle]

While trying to install dashHtmlComponents on a Windows instance, the following error was encountered:

This PR proposes to

• remove the symlink LICENSE which points to LICENSE.txt so that the R package will install properly
• add a LICENSE.md file which will properly render the MIT license within GitHub
• add a LICENSE file which contains the text required by CRAN when submitting packages (see https://cran.r-project.org/web/licenses/MIT)
• add LICENSE.md to .Rbuildignore to ensure package installs properly

If this arrangement is suitable for dash-html-components, it may be used for other repos in the future.

[rpkyle]

Closing, superseded by https://github.com/plotly/dash-html-components/pull/150.