MOTECH-589 User can select the actual CommCare fields in Tasks triggers

This commit changes the way Commcare module exposes task triggers.
Each form and case type has got a separate trigger now. This allows
to define actual fields present in a form or case for such triggers.
Also, JSON file is no longer used to register channel for the Commcare
module, since the CommCare Application schema may change and for each
such change we must build triggers from scratch. Instead, the Tasks
API is called to register channel for the Commcare module. Triggers
are built based on current application schema kept in MOTECH db.

Change-Id: If64c2bc06bcb3ab0561d3375eaecd2d4a0d9f7e6