Fix UDF choice editing helper function (hotfix)

[jwalgran]

NOTE: This is a copy of #3259 targeted at the 2.21.4 hotfix branch.

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 437b1d23d7c90cfdf9a283f0dd70a7ccf115453c on jcw/fix-udf-helper into d46202312fca4c36b1e74361699e7b544429bc9a on hotfix/2.21.4.

[jwalgran]

@azavea-bot rebuild