Item field is added to the pill items, this is done to utilize
it wherever possible, to distinguish between diffrent types
of pill items(stream|user_groups|users).
It is also a preparatory commit to support expanding of stream
and user group pills.
This extra commit adds support for creating user group pills
in cases that do not use typeahead like, pasting the group
name, copying it from the user group pill.
This completes the remaining work required to support
addition of all members of a user groups to stream.
Fixes#15186.
m-e-l-u-h-a-n