read-only chats are not really edge to edge

[adbenitez]

read-only chats are not really edge to edge but continue having the not-so-nice lower bar:

this is esp. visible at the very first impression of the device chat message. but also the new channels would look better if the wallpaper would continue to the edge.

(copied from #4064 (review))

[adbenitez]

due to some problems introduced by the PR fixing this issue, we needed to revert to the non-edge-to-edge behavior in #4219