Swap custom store for vuex implementation

...vascripts/merge_conflicts/components/diff_file_editor.vue

...ipts/merge_conflicts/components/inline_conflict_lines.vue

...ts/merge_conflicts/components/parallel_conflict_lines.vue

app/assets/javascripts/merge_conflicts/constants.js

...vascripts/merge_conflicts/merge_conflict_resolver_app.vue

...ets/javascripts/merge_conflicts/merge_conflict_service.js

...ssets/javascripts/merge_conflicts/merge_conflict_store.js

...ets/javascripts/merge_conflicts/merge_conflicts_bundle.js

...vascripts/merge_conflicts/mixins/line_conflict_actions.js

app/assets/javascripts/merge_conflicts/store/actions.js

..._conflicts/components/merge_conflict_resolver_app_spec.js

spec/frontend/merge_conflicts/mock_data.js