Fix UDF choice editing helper function

[jwalgran]

Overview

In the case where choice fields contain numeric values, this helper is called with a number and fails unless we force the value to a string.

Connects https://github.com/OpenTreeMap/otm-clients/issues/441

Testing Instructions

• Checkout the develop branch
• Browse http://localhost:6060/create/ and create an instance called 'numeric-choices'
• Visit http://localhost:6060/numeric-choices/management/udfs/ and click + Add Field
• Create a choice field like this and click Finish to save it.
• Refresh the page and click Edit. Start typing a new choice value and you will see an exception in the console
• Checkout this branch, reload the page, and click Edit.
• Verify that you can successfully add a new choice value and save it.

[coveralls]

Coverage remained the same at 83.796% when pulling 3604b8d4ff8f230838280b92afb08a6a0e50a8a8 on jcw/fix-udf-helper into cb1027c63c1dafabb5a26315fee6488f8042dd08 on develop.

[jwalgran]

Thanks. I am going to retarget this to the newly created hotfix/2.21.4 branch.

[jwalgran]

Dealing with an issue retargeting this to the hotfix branch. Will reopen.