Make activities link to the conversation

[nickvergessen]

Fix #3523

Before	After

In case we decide to backport this to stable18, the route needs to be changed.

[MartinKoeditz]

Just a hint:

I added your code change (the one liner) and now I receive the link. But it points to the nextcloud main page rather than to the room. Maybe it depends on my current version. Otherwise please have a look at that.

[nickvergessen]

Yeah that is the line bellow the image.

For Nextcloud 18 the line would need to be something like:
'link' => $this->url->linkToRouteAbsolute('spreed.pagecontroller.showCall', ['token' => $room->getToken()]),

[nickvergessen]

/backport to stable19