Add SRI hashes and local fallbacks for external resources

As discussed in PR #1 [Add location](https://github.com/VForWaTer/metacatalog-api/pull/1), it is suggested to add SRI hashes and local fallbacks for external resources to improve security and reliability. See the comment [here](https://github.com/VForWaTer/metacatalog-api/pull/1#discussion_r1834156767) for more details. This issue was requested by @mmaelicke.