feat: openfoodfacts#935 - now we display localized (downloaded) data …

…during onboarding

Deleted files:
* `sample_eco_knowledge_panels.json`: replaced by new file `sample_knowledge_panels.json`
* `sample_health_knowledge_panels.json`: replaced by new file `sample_knowledge_panels.json`

New files:
* `abstract_onboarding_data.dart`: Abstraction of data we download, store and reuse at onboarding.
* `dao_string.dart`: Where we store strings.
* `onboarding_data_knowledge_panels.dart`: Helper around knowledge panels we download, store and reuse at onboarding.
* `onboarding_data_product.dart`: Helper around a product we download, store and reuse at onboarding.
* `onboarding_loader.dart`: Helper around data we download, store and reuse at onboarding.
* `sample_knowledge_panels.json`: json data downloaded today for en_US, cf. onboarding_data_knowledge_panels.dart
* `tmp.dart`: Fixes to off-dart.

Impacted files:
* `knowledge_panel_page_template.dart`: added parameters to 1. use the database (and downloaded data) and 2. limit the display to a given panel
* `knowlegde_panel_builder.dart`: added an optional parameter to limit the display to a list of panels
* `local_database.dart`: added new dao `DaoString`
* `next_button.dart`: added an optional call to async methods at "next" time - e.g. data download
* `onboarding_flow_navigator.dart`: now using the database to get more relevant (previoulsy downloaded) data
* `preferences_page.dart`: now using previously downloaded data instead of assets
* `sample_eco_card_page.dart`: now using previously downloaded data instead of assets, and displaying only the environment card
* `sample_health_card_page.dart`: now using previously downloaded data instead of assets, and displaying only the health card
* `sample_product_json.dart`: json data downloaded today for en_US, cf. onboarding_data_product.dart