Final tidy up of subscription routes

Problem/Motivation

Simplenews has several routes with path starting with /newsletter/. This isn't great from a namespacing point of view (clash with a module called 'newsletter'), but also it's not logical or clear, as the routes mostly aren't especially related to newsletters. Many of the routes names aren't especially logical or clear either.

In 4.x we've already made several non-BC changes to these routes, so let's finish the job and get everything clear.

Proposed resolution

1. The path should start with /simplenews
2. The route name should start with simplenews.subscriptions_. There are already some unrelated simplenews.subscriber_ routes so we should avoid that.
3. The next part of the path/URL should match the action used in the hash keys: confirm, manage, add or remove.
4. It's clearer to have separate routes for add/remove rather than sharing a single one with a parameter

Remaining tasks

User interface changes

API changes

Data model changes