ORKStepHeaderView: refactor constraint code
ORKStepHeaderView: refactor constraint code

- Homogenize code.
- Move initial constraint creation to their own method.
- Add constraints by activation.
- Remove unneeded ivars.

jarrelscy/ResearchKit,KaiCode2/ResearchKit,md0u80c9/ResearchKit,YuanZhu-apple/ResearchKit,umerkhan-apple/ResearchKit,jarrelscy/ResearchKit,oliverschaefer/ResearchKit,oliverschaefer/ResearchKit,QuintilesRK/ResearchKit,mluedke2/ResearchKit,danoli3/ResearchKit,cornelltech/ResearchKit,danoli3/ResearchKit,shazino/ResearchKit,weijentu/ResearchKit,allen7wang/ResearchKit,boland25/ResearchKit,mluedke2/ResearchKit,chb/ResearchKit,md0u80c9/ResearchKit,jimmyti/ResearchKit,DrGo/ResearchKit,jimmyti/ResearchKit,boland25/ResearchKit,jarrelscy/ResearchKit,allen7wang/ResearchKit,weijentu/ResearchKit,allen7wang/ResearchKit,umerkhan-apple/ResearchKit,mluedke2/ResearchKit,QuintilesRK/ResearchKit,cornelltech/ResearchKit,umerkhan-apple/ResearchKit,md0u80c9/ResearchKit,chb/ResearchKit,KaiCode2/ResearchKit,KaiCode2/ResearchKit,jeremiahyan/ResearchKit,cornelltech/ResearchKit,danoli3/ResearchKit,jeremiahyan/ResearchKit,oliverschaefer/ResearchKit,boland25/ResearchKit,YuanZhu-apple/ResearchKit,weijentu/ResearchKit,shazino/ResearchKit,DrGo/ResearchKit,YuanZhu-apple/ResearchKit,md0u80c9/ResearchKit,jeremiahyan/ResearchKit,YuanZhu-apple/ResearchKit,jimmyti/ResearchKit,oliverschaefer/ResearchKit,boland25/ResearchKit,DrGo/ResearchKit,chb/ResearchKit,shazino/ResearchKit,weijentu/ResearchKit,QuintilesRK/ResearchKit,chb/ResearchKit