Add Prometheus memory sparkline to MR widget

...e_merge_request_widget/components/mr_widget_deployment.js

...merge_request_widget/components/mr_widget_memory_usage.js

app/assets/javascripts/vue_shared/components/memory_graph.js

app/assets/stylesheets/framework/memory_graph.scss

app/assets/stylesheets/pages/merge_requests.scss

app/controllers/projects/merge_requests_controller.rb

...pts/vue_mr_widget/components/mr_widget_deployment_spec.js

...s/vue_mr_widget/components/mr_widget_memory_usage_spec.js

spec/javascripts/vue_shared/components/memory_graph_spec.js

spec/javascripts/vue_shared/components/mock_data.js