Add an API for forwarding entities

Problem/Motivation

I'd like to leverage much of the goodness from forward.module when programmatically sending entities on my site. Currently, all of the code to actually render and send the email is buried inside the submit callback for a form.

Proposed resolution

Move nearly all the code in ForwardForm::submitForm() into a stand-alone API (perhaps in a new forward.send_entity service or something).

Remaining tasks

Decide on what it should all be called and where the code lives.
Move the code.

User interface changes

None.

API changes

Add new API for sending an entity.

Data model changes

None.

Release notes snippet

TBD.