Add a way to apply Subresource Integrity to CSS and JS resources automatically

**Is your feature request related to a problem? Please describe.**
Every time that GCWeb or WET-BOEW make a new release the integrity hashes need to be updated in the template.
 
**Describe the solution you'd like**
A JSON parser to parse the payload.json file with all the integrity hashes for GCWeb resources

**Describe alternatives you've considered**

**Additional context**
