notifications: Show stream names for private stream messages.
notifications: Show stream names for private stream messages.

Fixes #10745.

Use get_display_recipient to get stream names, and remove the
references to message.stream_name in push_notifications.py which were
added in 97571a203, as the actual stream names were being retrived
only for Message objects associated with public streams.

showell/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,dhcrzf/zulip,zulip/zulip,andersk/zulip,dhcrzf/zulip,dhcrzf/zulip,tommyip/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,jackrzhang/zulip,kou/zulip,brainwane/zulip,dhcrzf/zulip,rht/zulip,synicalsyntax/zulip,tommyip/zulip,synicalsyntax/zulip,rht/zulip,jackrzhang/zulip,kou/zulip,timabbott/zulip,timabbott/zulip,tommyip/zulip,eeshangarg/zulip,jackrzhang/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,rishig/zulip,punchagan/zulip,kou/zulip,timabbott/zulip,hackerkid/zulip,showell/zulip,kou/zulip,dhcrzf/zulip,shubhamdhama/zulip,brainwane/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,rishig/zulip,brainwane/zulip,dhcrzf/zulip,shubhamdhama/zulip,hackerkid/zulip,hackerkid/zulip,timabbott/zulip,tommyip/zulip,showell/zulip,hackerkid/zulip,rishig/zulip,rht/zulip,kou/zulip,shubhamdhama/zulip,andersk/zulip,brainwane/zulip,timabbott/zulip,eeshangarg/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,tommyip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,synicalsyntax/zulip,brainwane/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,jackrzhang/zulip,synicalsyntax/zulip,andersk/zulip,rishig/zulip,showell/zulip,shubhamdhama/zulip,rishig/zulip,punchagan/zulip,jackrzhang/zulip,punchagan/zulip,synicalsyntax/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,dhcrzf/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,tommyip/zulip,andersk/zulip,rishig/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,rht/zulip,timabbott/zulip,andersk/zulip,jackrzhang/zulip,rishig/zulip,andersk/zulip,rht/zulip,tommyip/zulip,jackrzhang/zulip,shubhamdhama/zulip