Add openmrs-core and Standalone 2.x module messages to transifex

Background

Currently we use the Transifex online tool to translate all of the Reference Application modules. But for translating openmrs-core and standalone we ask people to manually edit the messages_xy.properties files. Instead, we should use Transifex for openmrs-core and the standalone.

Acceptance Criteria

1. Transifex should have up-to-date latest versions of the openmrs-core and -standalone translations
2. openmrs-core and openmrs-standalone are configured to automatically pull in translations from transifex (in ci.openmrs.org)
3. Update wiki docs about translating OpenMRS Core to reflect using transifex.