This is a preparatory commit to set up basic UI for right panel
in user group settings overlay. At this point we only ensure
the proper display of the two panels under different screen sizs.
Actual functionality for user group creation and user group
edit will be added in subsquent commits.
Dedicated overlay for user group settings is added as part of
addressing zulip#19526.
The newely added overlay is currently empty and more UI
related to settings is to be added in further commits.
Purushottam Tiwari