continue refactor single-/multiple-selection table-question answer-schema & export

update schema to fit the following export format:

single-selection (radio)
  | question.code |
  |   choice.value |
multi-selection (checkbox)
  | question.code_choice.variable | question.code_choice.variable |
  |                                            1/0 |                                             1/0 |

remove choices.$.variable from single-selection (radio) and choices.$.value multi-selection (checkbox) questions
add migration to remove unused keys
update view to only show the corresponding field
sanitize choice.variable to choice.value for autoform options
autoform multiple-choice and table questions now store answers in the same format