Renaming start_date in order to date_activated

This has been done on master branch(will be backported to 1.11.x , 1.10.x).

For now, we have retained getters and setters for start_date which refer to newly added date_activated itself to support modules depending on older version of OpenMRS (1.9.x). But these methods are marked as deprecated methods and soon will be removed as we decide to upgrade/ not support the older versions.

- message.properties file for error messages relating to date_activated is only done for English language.