Add stores for data

[MaHaWo]

This is all somewhat tentative since I'm not proficient with web data management yet

what it does currently

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• goal: add svelte stores for children and content which can later be used to fetch and store data from the backend.
• use svelte-stores to store global data in and 'emulate' backend calls.
• There is no fixed API for yet for how these calls should be made in the end. Currently there's just some add , remove and fetch functions that operate directly on the prepopulated stores that contains only dummy data. For this reason, this PR doesn´t close the Dummy backend #10 yet.
  • 2 stores: children (contains userkey -> array of children objects, which are themselves dictionaries), content (contains data: surveys -> objects).
• currently there are no subscriptions because the stores do not change values at runtime yet.
• move dummy data defined in the components into the stores and use via the respective stores then.
• create a dynamic routing for children and surveys: Children summary page and survey pages are created dynamically depending on the data in the stores. This currently uses load to pass data from page to page --> is there something better or is this acceptable?
• add tests for functionality of stores where deemed relevant.
• use typescript in the xStore and xStore.test files (will slowly migate <script> blocks and .js files to typescript as things get refactored).
• there is no user store yet. This is because the user data management system is the least clear to me and the current login/register pages need deeper refactoring. For this reason, the child registration page does not make use of the childrenstore either.
• error handling is missing still.

Sorry this got so ugly, I went back and forth a lot on it...

[MaHaWo]

from here onwards in next PRs:

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• refactor registration and login pages such that one by one they get fully functional.
• add childrenregistration functionality first because the store is already there.
• add user store.
• add functionality to the user login/registration pages.

[MaHaWo]

@lkeegan Is there something to do here still?

[lkeegan]

lgtm