Short URL implementation

In preparation for SMS notifications, we need to make use of short URLs for channel redirection. We need to create a short URL on channel creation for those new channels and create the short URL on channel edit for legacy channels.

For removing, 2 options:

• Remove it when channel is removed
• Keep it for a period of time an have a job that removes them after X time

https://gitlab.cern.ch/push-notifications/notifications-design/-/issues/56#note_6583521