Follow up from "Several Fixes"

There are still several bugs and unwanted behavior:

• The session-id input field should only allow numbers and spaces
• The "answer-statistic-cards" needs some enhacement
• The text in the "back-button" in the header should be removed
• Adding the answers when creating the questions needs to be revised
• There are still icon-buttons which have weird sizes and shapes
• Check i18n: footer, login, statistic-help
• Currently its not possible to join a session if user is already logged in as guest
• Participants comment buttons should have a min-size
• The editing of content needs some overhaul
• A click on the ARSnova-title should navigate to home
• The participants content-carousel should be fixed in the top and the buttons should be centered