Merge branch '160-do-not-show-the-registration-form-but-let-the-session-creator-continue-as-a-guest' into 'master'

Resolve "Do not show the registration form, but let the session creator continue as a guest."

Closes #160

See merge request arsnova/frag.jetzt!149

Added fullscreen to toggle button

Closes #150

See merge request arsnova/frag.jetzt!148

Resolve "Center the Session dialog vertically"

Closes #146

See merge request arsnova/frag.jetzt!146

Allow the user to use only if he has accepted cookies and privacy protection

Closes #142

See merge request arsnova/frag.jetzt!147

Resolve "Freeze comment stream"

Closes #116

See merge request arsnova/frag.jetzt!144

Show voting-score moderators too

Closes #139

See merge request arsnova/frag.jetzt!145

changed background color to beige, changed surface color to white to make mat-slide more visible

Closes #145

See merge request arsnova/frag.jetzt!143