Resolve "content types (HTML)"

Closes #110

See merge request swtp-block-ws17/arsnova-angular-frontend!89

Add route to question carousel for participant

Navigate to new content creation page when clicking on "Create new content"
Make ContentCreation component a component (not a MatDialog)
Add new routes to new pages

Resolve "answer statistics (template)"

Closes #112

See merge request swtp-block-ws17/arsnova-angular-frontend!82

Resolve "content types (classes)"

Closes #109

See merge request swtp-block-ws17/arsnova-angular-frontend!86

Select first content (due to array as return paramter)

Remove unnecassary api call for room to get room id (?!)

Re-arrange AnswerOptions (code style)
Add constructor for content
Refactor ChoiceContent
Refactor TextContent