Adjust gaps between chat page edges and messages

* Insert a gap between the oldest message and the top edge of the chat page.
* Reduce the gap between the own messages and the right edge of the chat page.
3 jobs for design/chatpage-gaps in 2 minutes and 31 seconds (queued for 161 minutes and 31 seconds)
Status Job ID Name Coverage
  Test
passed #74819
build

00:01:03

passed #74817
clazy

00:01:06

passed #74818
qmllint

00:00:20