Add tests and improve new tasks typing

[correct-horse-battery-bench]

During manual testing and writing tests I noticed a few dead code paths and typing improvements which allowed to remove / simplify some redundant code. Regarding typing, some things here I think are essential and some changes are opiniated. Happy to discuss with whoever reviews this pr.

Main Changes:

• Use form.controls.<name> instead of form.get in order to have stricter typing
• Remove whichView and copyType, isCopyHashlist dead variables not used anymore in any code path, whichView previously was 'create' or 'edit' but there seems to exist now an edit-task page dedicated for that. Type the kind passed by the router as data
• use takeUntilDestroyed instead of UnsubscriptionService
• use firstValueFrom and await if possible
• Generally add some more restricted typing

[correct-horse-battery-bench]

Riskiest change of the pr and worth to have a look at.
When copying a pretask the value '9999' was used as placeholder there so that the input-multiselect is still shown when copying a pretask which is semantically incorrect as pre tasks do not seem to have an assigned hashlist.

Generally:
If new task -> always show this (!copyMode)
If we copy a pretask/task we still want to show this input as all tasks seem to need one

The !isCopyHashlistId seems to just check if we load a task that the hashlistId of it seems to be loaded (as default null). But the form is patched anyways so it seems to me that this check is not needed anymore...