Drop deprecated API from escalus_users

An Xref analysis of ejabberd_tests shows that none of the suites
in default.spec use this API so I consider it safe to remove.
Rename: short_option -> user_option
Error out when no event manager is found in Config

This should be redesigned not to rely on Common Test config.
Event history can be stored in the #client{} record
or in a process pointed at from the record.
Finish speccing escalus_users (w/a few coupled external funs)

Dialyzer is happy now! Don't foul it.
Remove escalus_users:get_users/1 passthrough clause

This is a backwards incompatible change,
but there are no use cases outside Escalus.
Make escalus_ejabberd comply with escalus_user_db behaviour

This drops some API functions, hence is a backwards incompatible change.
Streamline user creation in escalus_users
Cleanup escalus_users types
Drop explicit type cast - rely on specs and sane caller

There are no use cases in tests according to Xref - this removal should
be safe.
This is an API change - consider changing the version number.
Enable strict (warnings as errors) Dialyzer check on Travis