Resolve "Add enterprise license support"

What is in this MR:

Refactor:

1. Introduce a LicenseHandler class to group all the license methods into one class to be consistant with our other handlers + make method discovery easier
2. Add a new LicensePlugin class which allows modification of the license system.
3. Add a new plugin in the frontend which lets you write code like this.$hasFeature(PremiumFeatures.PREMIUM, this.group.id)
4. Changes the license data structure sent from the backend to the frontend and stored in the auth store to be:

{
   active_licenses: {
        instance_wide: {
              license_type_1: true,
              license_type_2: false,
        },
        per_group: {
             1: {
              license_type_1: true,
              license_type_2: false,
             }
       }
}

Closes #1230 (closed)