Added google consent mode functions

Done

Clone the repo
Build and run the demo
```
$ yarn build
$ yarn serve
```
Check that before selecting cookie preferences, the <head> section of the site has the default gtag consent values (all denied)
Check that after selecting cookie preferences i.e functionality or performance, the gtag values are updated correctly in the <head> section.
- They should correctly match the essential, functionality or performance selections, as specified here
Test on any site that uses the cookie-policy. canonical.com is a good example
- In the cloned repo, copy the static/js/modules/cookie-policy/cookie-policy.js file to static/dist/