Get rid of rxdart

We should get rid of `rxdart` as a dependency - it is causing a lot of dependency problems on a regular bases - like this one:

```
Because research_package >=1.7.1 depends on rxdart ^0.28.0 and oidc >=0.2.0+1 depends on rxdart ^0.27.7,
research_package >=1.7.1 is incompatible with oidc >=0.2.0+1.
And because carp_webservices >=3.3.0 depends on oidc ^0.7.0, research_package >=1.7.1 is incompatible
with carp_webservices >=3.3.0.
So, because carp_study_app depends on both carp_webservices ^3.5.0 and research_package ^1.7.1, version
solving failed.
```

And it should be doable to make our own reactive model w/o using rxdart.