Use HTML5 form validation for inputs in feature flag manage

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The following discussion from !14538 (merged) should be addressed:

• @pslaughter started a discussion: (+4 comments)

  nitpick: Now that we're using an HTML5 element, we can probably use their native validation API.

  isRolloutPercentageInvalid() {
    return this.$refs.rolloutPercentageInput.checkValidity();
  },

  If we do this, we should probably move it to a computed for caching 👍