Resolve "login screen - service" 2nd

See merge request swtp-block-ws17/arsnova-angular-frontend!58

Don't return observables in getter functions anymore as we're storing the user in a object we can return immediately.

Store and load user data so we don't lose the user's data on reload.

Added service handling local data. Use this service so we can change the used technology (cookie, local storage, ..) easily.

Resolve "HttpService - Error handling"

Closes #78

See merge request swtp-block-ws17/arsnova-angular-frontend!52

Resolve "comment creation - creator component logic"

Closes #41 and #27

See merge request swtp-block-ws17/arsnova-angular-frontend!54

Resolve "creator/participant differentiation (login)"

Closes #76

See merge request swtp-block-ws17/arsnova-angular-frontend!42

Resolve "room creation - Logic"

Closes #26

See merge request swtp-block-ws17/arsnova-angular-frontend!48