Align license compliance `deny` classification outcome with user expectations - Prevent MR from being mergable in UI

See: #196845 (closed)

Don't allow MR to be merged if any license policies have classification as denied. This would mean that for a particular license in the Policy tab that has a rule of Denied that is then detected in the projcect.

##Implementation:

See #196845 (comment 336410150) for a complete code walkthrough:

What we need to do:

1.) Add a new state.key in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/stores/get_state_key.js

2.) We need to add a new mapping in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/vue_merge_request_widget/stores/state_maps.js

3.) Create the new Vue component called mr_widget_policy_violation and put it in https://gitlab.com/gitlab-org/gitlab/-/tree/master/app/assets/javascripts/vue_merge_request_widget/components/states

4.) Write unit tests