Meta-schema improvements

* Forces link hrefs to match our format of an absolute path beginning with a "/" and only containing lowercase letters and hyphens _unless_ the clause is a JSON reference.

    ```
    ../api/docs/v3/schema.json: #/definitions/config-var/links/0/href: failed schema #/definitions/link/properties/href: Expected string to match pattern "/^(\/([a-z][a-z-]*[a-z]|{(.*)}))+$/", value was: /apps/{(%23%2Fdefinitions%2Fapp%2Fdefinitions%2Fidentity)}/config_vars.
    ```

* Forces resource properties (i.e. like `name` on `app`) to use only lowercase letters and underscores.

    ```
    ../api/docs/v3/schema.json: #/definitions/domain/properties: failed schema #/definitions/resource/properties/properties: Extra keys in object: created-at.
    ```

* Mandates that resource properties are either references (for a basic property like `name` under `app`) to definitions or a subobject themselves (for say `owner` under `app`).

    ```
    ../api/docs/v3/schema.json: #/definitions/domain/properties/hostname: failed schema #/definitions/resource/properties/properties/patternProperties/^[a-z][a-z_]*[a-z]$: Data did not match any subschema of "anyOf" condition.
    ```

* Forces all resources to define `strictProperties` to `true` (and all subschemas contained in `properties` too).

    ```
    ../api/docs/v3/schema.json: #/definitions/account-feature/strictProperties: failed schema #/definitions/resource/properties/strictProperties: Expected data to be a member of enum [true], value was: false.
    ```

/cc @geemus @heroku/api