The hex2rgb function seems to previously have been available globally.
It probably got lost in the great Webpack refactoring and nobody noticed
that.
This copies the function into its own file (to make importing it easy)
and adds an import in index.js.
Fixes https://github.com/hedgedoc/hedgedoc/issues/2248
Signed-off-by: David Mehren <git@herrmehren.de>
