Move subscribe logic to a service

Problem/Motivation

When using this module we sometimes need a more complex form that the one provided so we build a custom form but we would still like to reuse the subscribe logic.
Moving subscribe() to a service would make it easier to reuse and reduce code duplication.

Steps to reproduce

Move SignUpForm::subscribe() to a new service.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes