12624: xlsform export: fix duplicate column header error in Central

[plastichotsprings]

if a form references 2 different option sets, and the option sets have a duplicate level name, ODK Central will fail to upload the generated XLSForm with the error: the XLSForm could not be converted: Duplicate column header

To fix this, we prepend the option set name (which is unique in NEMO) to the level name whenever it's referenced in the XLSForm.

[cooperka]

nice and straightforward. thanks for mentioning that the os_name is guaranteed unique.

tested and verified in ODK Central! while testing i did have to add the allow_choice_duplicates setting to allow big cascading selects, e.g. where "Main St" might be a street that exists simultaneously in multiple unique cities.

i also created unique_level_name so there's only 1 method that generates these level names since it's being done 3 times. wouldn't want to later make a tweak that changes 2/3 and forget the third.

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ **NameError: undefined local variable or method constraint_message_to_push' for an instance of Forms::Export ...** FormsController#export_xls` View Issue

_{Did you find this useful? React with a 👍 or 👎}