Support most common greenlight urls

[SamuelWei]

Is your feature request related to a problem? Please describe.
With #117 the rooms can be imported. The room ids stay the same, so using some redirects the old urls could be supported.
To support the room urls, signin, default room, etc. nginx rules can be used.

Describe the solution you'd like
To make the transition as easy and smooth as possible, a greenlight compatibility would be helpfull.
It would catch the old greenlight urls and redirect to the PILOS equivalent if it exists.

Describe alternatives you've considered
Just provide a support file with nginx redirect rules.