fix(validation): fix rules and unit tests

[NozomuIkuta]

Fix #437.

TODOs

• [x] Add optional: true flag to rules that need it
  • [x] Replace Vuelidate's helpers.req() with internal code
• [x] Fix validators as needed
• [x] Restore and add missing unit tests of validators
  • This is because we want to use validators independently from validation rules
• [x] Add unit tests of validation rules

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