Migrate Alerting Kibana pluign to the new Kibana Platform

[ftianli-amzn]

Alerting Kibana plugin is developed with the "legacy plugin" API of Kibana, and the support for "legacy plugin" has been removed in Kibana 7.10. Alerting Kibana plugin needs to be migrated to the new Kibana platform.
https://www.elastic.co/blog/introducing-a-new-architecture-for-kibana
https://www.elastic.co/guide/en/kibana/7.10/breaking-changes-7.10.html

All the tasks are tracked here.

• Migrate all the features of the plugin #209
• Review public/less/main.less and migrate the necessary parts to SCSS
• Update TRANSLATION.md
• Review the "scripts" in package.json after the plugin is compatible with Kibana 7.10