Mailer classes don't need to be plugins

Problem/Motivation

Currently a mailer plugin is both a service and a plugin. The plugin part is only really used for collecting definitions. It is also currently used as a factory for creating the service, but it doesn't need to be like that.

Proposed resolution

1. In SymfonyMailerServiceProvider instead of setting a factory, add a tag mailer_instance
2. Change the service file to name the mailer class in the normal way
3. Rename MailerPluginBase to ComponentMailerBase and remove the extending of PluginBase
4. Change MailerManager to Discover the mailers by means of the tag

Remaining tasks

User interface changes

API changes

Data model changes