ui: Improve message rendering spacing and avatar size
ui: Improve message rendering spacing and avatar size

After analyzing several competitors and specifically Slack I noticed
all our paddings are very small (factor of 1.5 to 2, but mostly 2)

This increases the padding between the message and the borders, and
makes the avatar size 10% smaller from 2.5em (37.5px) to 32px to
be consistent with the avatars outside the message list (and Slack)

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile