Adds "continue if empty" flag to UI for fields

- Should fix an issue reported by @dstockto - essentially, setting the following
  combination:

    - required: true
    - allow empty: true
    - continue if empty: true

  will force validations to be run for that input (instead of skipped, which is
  what happens when continue if empty is false but the other two are true).
Rebuilt distribution
Merge branch 'hotfix/fields-continue_if_empty-flag'

Adds the "continue if empty" flag to the Fields UI.