populate_db: Suppress logging output for push notifications.
populate_db: Suppress logging output for push notifications.

This fixes a lot of spammy output of the form:

2018-11-27 17:46:48.279 INFO [zerver.lib.push_notifications] Sending push notification to user 46

when running populate_db, which is both confusing (since we're not
actually sending push notifications here) and spammy.

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