Merge branch '52-login-screen-login-data-model-user' into 'master'

Resolve "login screen - login data model (user)"

Closes #52

See merge request swtp-block-ws17/arsnova-angular-frontend!8

src/app/user.ts

+export class User {
+  id: number;
+  name: string;
+  email: string;
+  isCreator: boolean;
+}