[wip] [skip ci] Notification plugin migration

[peterschretlen]

Summary

Migrating the notifications plugin to the new platform.
At the moment this still runs as a legacy plugin, but shims have been put in place to make it ready for migration to new platform services.

[elasticmachine]

Pinging @elastic/kibana-platform

[rudolf]

There's a few places where the old JS code had JSDoc comments with type information like @param {NotificationService} notificationService The notification service singleton. Since typescript now takes care of the types we can remove the {NotificationService} type specifier from these JSDoc comments.

The (relatively new) TSDoc standard requires that there's a - between paramater names and their description so the above comment would become:
@param notificationService - The notification service singleton.

We don't parse the TSDoc comments in plugins, so it's not a strong requirement to stick to the spec.

[epixa]

I'm going to close this as I believe we're ready to remove the notifications plugin entirely in favor of the new actions plugin. Thanks for doing this exercise nonetheless!