add JoinRequest model
add custom permission to Community model
list community permissions and groups
create groups according to community name + unittest
add django-guardian package for row-level permissions
assign permissions to community groups + unittests
SystersUser join and leave group + unittests
move groups templates to constants
signal to manage community groups definition
store original name and community admin to the instance + unittests
add remove groups util function + unittest
add Community methods to check if name or admin changed + unittests
implement manage_community_groups + unittest