require `titles.title` in conferences

[michamos]

the titles field in conferences is not required, but the UI assumes it is. In practice it's always present, so we should change the schema to require it.

[drjova]

TECHNOTES:

1. Add titles as a required field here
2. Update the tests to cover this case.

ACCEPTANCE:

• When the user submits a conference without a title it should fail with validation error.
• When the developer create or update a conference via the API and doesn't have title it should fail with validation error.
• When an applicaation create or update a conference via the RESTAPI and doesn't have title it should fail with validation error.

[drjova]

Get the control_number of the conferences without a title, use ES query.