closes #3303 RFC: Rename Validator::between() into Validator::length()
replaces validator 'between' with 'lengthBetween'
fixes falling test case for 'lengthBetween' validator
adds backwards compatible 'between' alias for 'lengthBetween' validation rule
fixes model task test-cases for 'between' validation rule alias
Merge pull request #3478 from Schlaefer/#3303-lengthBetween

closes #3303 RFC: Rename Validator::between() into Validator::length()