Resolve "Content Types"

Closes #159 (closed)

Proposed changes

• Add content types for
  • single / multiple choice
  • likert choice
  • yes / no choice
• Add creator's and participant's view and logic

Testing instructions

As creator:

• login, join room, create content - you can check the correct output with a console.log of this.content in each submit-method in the component's *.ts file
• try to make mistakes during content creation and check if the notifications are correct.

As participant:

• login, join room, answer the dummy content - you can check the correct output with a console.log of this.content in each submit-method in the component's *.ts file
• try to make mistakes during answer selection / creation and check if the notifications are correct.

Not working

• api-calls
• content-list-component - without working api it is not possible to implement it. Router-links are not reachable