Standalone `gl-form-radio` components do not work without a `gl-form-radio-group` ancestor

As of writing, our gl-form-radio component wrapper is unusable. Once !192 (closed) is merged, it will be usable, but only as a descendant of a gl-form-radio-group component.

The underlying b-form-radio components we wrap can be used without any ancestor b-form-radio-group component, however. The issue is that we're not correctly forwarding the v-model directive from the gl-form-radio component down to the b-form-radio.

Part of #135 (closed).