Sanitize transform: apply data sanitizing steps

[osahon-okungbowa]

TASK TACKLED:

• handled all sanitising tasks listed here AND more here

[nightsh]

I added a couple of tweaks:

• only create the _clean_data property if there's a different value to save (i.e. don't create it if there's nothing changed)
• indent the JSON (pretty print) when saving :slightly_smiling_face: