Improve the way of opening attached pictures (and videos)

[pilsnerbeer]

Is your feature request related to a problem? Please describe.
If someone sends you a picture in the chat and you click it, you are redirected to the "files" from where you view the picture. Same for video files. You get a small thumbnail showing you what the picture is but to view it in full, you have to go through files.

Describe the solution you'd like
The workflow would be much faster if the user was able to view pictures from within the chat, that is - click the picture and it appears on the screen (hovering above the chat). Close it and continue chatting. For videos, render a thumbnail in the chat area (currently no thumbnail is visible) and when clicked, play the video as an overlay the same way.

Some quick designs of what I mean:

[nickvergessen]

We plan to use the viewer app from the files list after #1811 and #1347

[szaimen]

nextcloud/viewer#271 is merged so the viewer app is ready now. :)

[nickvergessen]

#1811 is in, and #2778 is on the way