Add SRI checking, requests resources from the same domain, fixed soon to be broken RawGit link

[Richienb]

The following changes have been made in this Pull Request:

• All external resources are now called from the same domain
• All external resources are now updated to the latest compatible version
• All external resources now use SRI checking
• As RawGit will shut down in late 2019, the URL was replaced

[BenRoe]

Please merge.