Resolve "navigation (main pages)"

Closes #107

See merge request swtp-block-ws17/arsnova-angular-frontend!101

Resolve "Differentiate between guests and registered users"

Closes #166

See merge request swtp-block-ws17/arsnova-angular-frontend!99

Resolve "navigation (login/logout)"

Closes #108

See merge request swtp-block-ws17/arsnova-angular-frontend!98

Resolve "Refactor project structure"

Closes #74

See merge request swtp-block-ws17/arsnova-angular-frontend!97

Rename identifiers

Rename models

Remove content-create

Remove unwanted imports and functions
Fix tslint