Split up MessageMetadataHelper and avoid duplicating its functionality in ConversationWrapper.
Use new MessageMetadataHelper in a few more places.
Fix url token message sensitivity.
Undo silly change.
Use a helper instead of building messages manually.
Merge branch 'feature/adhoc-mdh-improvements-from-821' into develop
Merge branch 'develop' into feature/issue-821-unified-message-storing-middleware

Conflicts:
	go/vumitools/conversation/tests/test_utils.py
	go/vumitools/conversation/utils.py
	go/vumitools/tests/test_utils.py
	go/vumitools/utils.py