tests(e2e): added prepopulation support; fixed login and users specs

yes:

```sh
env FRONTEND="https://test.sd-test.sourcefabric.org" BACKEND="https://test.sd-test.sourcefabric.org/api/" ./node_modules/.bin/protractor --baseUrl="$FRONTEND" --params.baseBackendUrl="$BACKEND" protractor-conf.js
```