[pgAdmin][RM-5404]: Show the login roles that are members of a group role be shown when examining a group role.

Hi Hackers,

Please find the attached patch for RM-5404
<https://redmine.postgresql.org/issues/5404>: Show the login roles that are
members of a group role be shown when examining a group role.
Updated the existing "Role" label to "Member of" and added a new option
"Members" to show all the roles that are members of the current Role/Group.
Users can add or remove the roles from the list similar to the "Members of"
option.

Also added 2 UI fixes:
1. "Member of" (Roles in earlier implementation) and "Members" fields
(Membership section) should be read-only in the properties panel.
2. If the select2 controller is set to read-only its background color is
not getting set as per the theme color. (earlier it is set to '#eee' for
all themes.)

--
*Thanks & Regards,*
*Nikhil Mohite*
*Software Engineer.*
*EDB Postgres* <https://www.enterprisedb.com/>
*Mob.No: +91-7798364578.*