Added a user_list_style personal user setting to the bottom of
Settings > Display settings > Theme section which controls the look
of the right sidebar user list.
The radio button UI includes a preview of what the styles look like.
The setting is intended to eventually have 3 possible values: COMPACT,
WITH_STATUS and WITH_AVATAR; the final value is not yet implemented.
Co-authored-by: Tim Abbott <tabbott@zulip.com>
