Add Apache 2.0 license to the repository to initalize it.

The Shivering-Isles Keycloak theme, will be derived from the official
Keycloak theme. Therefore using the same license is essential.

This commit also has the nice ability to allow easy rebases of the theme
to newer Keycloak versions. Since it's the initial commit, and the
template will be commited on top, this will never cause unrelated source
trees within the same repository.