refactor `api.js`

[duncdrum]

api.js currently tests the connection dialogue (with additional feature tests ontree_spec.js) and api compatibility.

1. the current api tests should become unit tests, and be replaced by a check for the visibility of the warning pop-up within cypress as api_spec.js.
2. the UI tests for the connection dialogue need to be refactored into a new connection_spec.js .
3. this should be the only place where UI based login happens, all other connections should be established via inbuild methods with fixtures
4. we might want to add additional unit tests for the connection functions

[duncdrum]

close by #514 further changes should be new issues