Convert anchor tag to pills containing an image/avatar
and username within both existing and new stream
subscriber member lists and group member lists
using the new user_display_only_pill.hbs template.
If user has long username that breaks the UI,
overflow is handled with ellipsis.
add a context class (panel_subscriber_member_list)
to add 'padding 5px'to 'pill_value' for stream
subscriber member list and group member list.
To retrieve an avatar of a user add a
'img_src: people.small_avatar_url_for_person(user/person)'
field in 'stream_editsubscriber.js', 'user_group_edit_members.js',
'stream_create_subscribers.js' and 'user_group_create_members.js'.
Fixes: #25724.
sujal