portico-header: Add background to avatar images.
portico-header: Add background to avatar images.

This adds a white background color to avatar images that may
have a transparent background.

hackerkid/zulip,timabbott/zulip,eeshangarg/zulip,eeshangarg/zulip,synicalsyntax/zulip,mahim97/zulip,mahim97/zulip,rishig/zulip,tommyip/zulip,dhcrzf/zulip,kou/zulip,showell/zulip,hackerkid/zulip,jackrzhang/zulip,hackerkid/zulip,kou/zulip,rht/zulip,showell/zulip,jackrzhang/zulip,tommyip/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,rishig/zulip,synicalsyntax/zulip,punchagan/zulip,synicalsyntax/zulip,mahim97/zulip,showell/zulip,rht/zulip,hackerkid/zulip,tommyip/zulip,hackerkid/zulip,timabbott/zulip,shubhamdhama/zulip,synicalsyntax/zulip,brainwane/zulip,brainwane/zulip,eeshangarg/zulip,jackrzhang/zulip,kou/zulip,tommyip/zulip,andersk/zulip,shubhamdhama/zulip,rht/zulip,eeshangarg/zulip,dhcrzf/zulip,rht/zulip,zulip/zulip,punchagan/zulip,jackrzhang/zulip,shubhamdhama/zulip,dhcrzf/zulip,dhcrzf/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,timabbott/zulip,synicalsyntax/zulip,dhcrzf/zulip,kou/zulip,shubhamdhama/zulip,rishig/zulip,dhcrzf/zulip,tommyip/zulip,mahim97/zulip,eeshangarg/zulip,rishig/zulip,mahim97/zulip,brainwane/zulip,timabbott/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,shubhamdhama/zulip,andersk/zulip,showell/zulip,andersk/zulip,brainwane/zulip,timabbott/zulip,brainwane/zulip,showell/zulip,punchagan/zulip,tommyip/zulip,showell/zulip,kou/zulip,mahim97/zulip,synicalsyntax/zulip,zulip/zulip,zulip/zulip,rht/zulip,dhcrzf/zulip,punchagan/zulip,shubhamdhama/zulip,shubhamdhama/zulip,jackrzhang/zulip,kou/zulip,timabbott/zulip,punchagan/zulip,rishig/zulip,rishig/zulip,punchagan/zulip,tommyip/zulip,jackrzhang/zulip,andersk/zulip,rishig/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,jackrzhang/zulip,timabbott/zulip