Resolve "Bug: unknown validator: phone "

What does this MR do and why?

The reason should be the api file jh/lib/jh/api/users.rb is loaded before jh/config/initializers/grape_validators.rb, but I can NOT reproduce the unknown validator error on my local even mocked as production.

Let's see if we can fix this after putting register_validator line to that api file.

• Move custom grape validator to api file

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Related to #1559 (closed)