feat: new data structure and database storage

[fpervaiz]

This PR makes major changes to the app frontend and backend.

• The top level item is now a Household rather than a GibsonsForm (collection). Respondents are nested inside Households and GibsonsForms nested inside each respondent. See the diagram below. (Survey level will be implemented in a future PR)
• New screens for creating and editing Households and Respondents have been added.
• All above data is now stored in an Isar database instance rather than SharedPreferences.
• Import/export code has been updated to maintain compatibility with legacy CSV export format.

There are some outstanding bugs to fix and a lot of cleanup to do however this will be done in subsequent, smaller PRs rather than this one as this PR is already very large.

[fpervaiz]

Known issue: household ID clashes are not handled when creating a new household. Reference in new issue upon merge.

[fpervaiz]

Known issue: when a household or respondent is edited and saved, the view household/respondent page doesn't update with new values unless going back and opening the household/respondent again. Reference in new issue upon merge.