Better Calendar support

[TCA-Bot]

Would be great to have the calendar on the same page, could probably be incorporated easily with an iframe tag.

[Marius1501]

@CommanderStorm Should a calendar be implemented on every site with a map or how should I understand this? 😄

[CommanderStorm]

No, a calendar only exists for some rooms like https://nav.tum.de/room/5510.02.001.
Currently this is a link to TUMOnline, this issue would like to see this being transformed into a modal with this calendar in an iframe.

Longer term integrating the calendar into our app properly would be advantageous, but I think this is some of the work we should push after #200 for the workflow improvements this PR will bring.
For this we will need to add an API route and add a component.

[CommanderStorm]

Update: we now have a calendar API for accessing calendar data see https://nav.tum.de/api for further details
This change schould be implemented upon the more modular webclient of #200