refactor: migrate rating validator to ts

[tshuli]

Problem

Part of #7

Solution

Improvements

• Migrates rating validation to Typescript

Tests

• Create two rating fields. Hide one of them behind logic.
• Check that validation on both visible and hidden rating fields work by submitting programatically
• Rating is accepted if it is an integer between 1 and "steps", rejected otherwise
• Rating must not have trailing zero

[karrui]

lgtm