Fixes#4217 Adds notifications
- Created a class to define notification-objects, which should notify other persons editing the same motion.
- Added functions to send notifications, listen to them and unsubscribing them.
- Added a warning-function to the `base-view.ts`, which raises the snack bar with the given message and has no duration.
Fixes#4217
- Removed unnecessary lines of code.
- Fixed merge.
Prettified
- Added a random number to identify different user.
- Now the user can sign in as the same user, but still receive a message if multiple people edit the same motion.
Fix the detail of motions
- If the user does not click to edit, then the `editNotificationSubscription` was not set.
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