tech debt: Change idoms in var names and funcs from "face" to "img"

[boazsender]

This is a tool for image annotation, not face annotation.

[boazsender]

"workloadId":1,
"images":[
  {
    "id":1,
    "annotations":[
      {"name":"Perceived Age","value":"adult"},
      {"name":"Perceived Gender","value":"androgynous"},
      {"name":"Perceived Ethnicity","value":"white"}
    ]
  },{
    "id":2,
    "annotations":[
      {"name":"Perceived Age","value":"young adult"},
      {"name":"Perceived Gender","value":"male"},
      {"name":"Perceived Ethnicity","value":"black"}
    ]
  }
  ...{the last object}
]}"