Burotel: Disallow invalid electronic-lock values

People put "on", "Yes", etc. and none of these values work, since it has to be literally "yes".

It would be nice if we failed validation when a value for that attribute is provided but anything but "yes".
To disable they should remove the attribute altogether, so no need to handle "no" etc.

Locate the electronic-lock attribute handling and its validation entry point in the CERN-specific Indico plugin. Confirm how provided values are currently processed, then make validation accept only the literal "yes" and treat an omitted attribute as disabled; verify the invalid-value and omitted-value cases with the plugin's existing checks.

Written by the indexing model from the issue text.