fixes https://github.com/TryGhost/Team/issues/2308
- Still has some missing pieces, but mostly works.
- Uses new handlebars template for emails
- When sending emails with the new email stability flag enabled, one
test email is now sent via the default smtp ghost mailer.
fixes https://github.com/TryGhost/Team/issues/2282
Added a new email service package that is used when the email stability
flag is enabled. Currently not yet implemented so will throw an error
for all entry points (if flag enabled).
Removed usage of `labs.isSet.bind` across the code, because that breaks
the stubbing of labs by `mockManager.mockLabsEnabled` and
`mockManager.mockLabsDisabled`. `flag => labs.isSet(flag)` should be
used instead.
All email depending tests now disable the `emailStability` feature flag
to keep the tests passing + make sure we still run all the tests for the
old flow while the email stability package is being built.
