Change node-sass to dart-sass

[spfaffly]

Currently we receive a lot of problems with errors on 'node-sass bindings'. This is caused when a node-sass binary is downloaded in the docker image (Linux) and then ran outside on the host machine (Mac OSx).

/Users/spfaffly/projects/unitedrentals/app/node_modules/node-sass/lib/binding.js:15
throw new Error(errors.missingBinary());
^
Error: Missing binding /Users/spfaffly/projects/unitedrentals/app/node_modules/node-sass/vendor/darwin-x64-64/binding.node
Node Sass could not find a binding for your current environment: OS X 64-bit with Node.js 10.x
Found bindings for the following environments:

• Linux 64-bit with Node.js 12.x

If we change the gulp process to use the dart-sass version this alleviates the issue.