Validate schema when allowing custom fields to be edited

[AritroSaha10]

When we're allowing custom fields to be edited, make sure to also enforce the schema. For example, it's possible to update an integer field to a decimal, which is not so great. It's not a fatal flaw right now, but when we expand to include more types like timestamps, it'll probably become a much larger issue if someone accidentally somehow edits it to a different type and the main ticket page breaks due to bad type conversion.

Low priority since I can only see this becoming a big issue for Haunted House