updated UI
Apply Model and concept changes to WebRTC implementation:
- participant information now contains:
  - user token
  - client ID
  - both combined to identifier
  - and name

basic error handling -> use only audio, when no video is available.

minor adjustments in view.
removed scaffolds.css.scss -> destroys layout
* changed integer for startDate to DateTime
* adjusted validation of meeting time
* adjusted _form for new meetings
modified migration to convert dates in order to avoid reset.
Merge branch 'master' into development
WEB-58 – typo