Implement support for all core field types at the entity level

So far subscriptions_content.module has implemented Mail Editor token support for file and term reference fields in nodes.

I want to move this support from the node level to the entity level, so that it is available for entity types other than nodes, if anyone wants to implement subscribing to such entities.

Besides [subs:terms:?] and [subs:files:?] I want to implement [subs:texts:?], which covers all field types by default.

The advantage of accessing fields through the [subs:...] tokens over the mechanisms in the Token and Entity Token modules is that we can count the values and iterate over them, so that we can control how they are rendered.

[subs:texts:?] directly accesses the first value as a convenience for single-valued fields — no need for [subs:texts:?:first] in this case.