build_emoji: Fix the spritesheet CSS generation code.
build_emoji: Fix the spritesheet CSS generation code.

Modify the spritesheet generation code to account for the differences
between emoji_map.json and iamcal's dataset. Due to the differences
between the two mappings, some emojis like :one:, :two: etc were not
getting rendered properly in the two emoji pickers where we are using
the iamcal's spritesheets for rendering them. This was so because there
was no CSS class corresponding to the codepoints of these emojis(as
mapped using emoji_map.json) in our spritesheet CSS(generated using
iamcal's dataset).

Fixes: #4775.

hackerkid/zulip,timabbott/zulip,jrowan/zulip,amanharitsh123/zulip,jphilipsen05/zulip,j831/zulip,dhcrzf/zulip,jrowan/zulip,jackrzhang/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,rht/zulip,brockwhittaker/zulip,tommyip/zulip,mahim97/zulip,rishig/zulip,brockwhittaker/zulip,verma-varsha/zulip,Galexrt/zulip,showell/zulip,rht/zulip,eeshangarg/zulip,jrowan/zulip,amanharitsh123/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,synicalsyntax/zulip,dhcrzf/zulip,j831/zulip,christi3k/zulip,tommyip/zulip,andersk/zulip,shubhamdhama/zulip,shubhamdhama/zulip,kou/zulip,mahim97/zulip,brockwhittaker/zulip,dhcrzf/zulip,vabs22/zulip,punchagan/zulip,brockwhittaker/zulip,brockwhittaker/zulip,tommyip/zulip,synicalsyntax/zulip,andersk/zulip,brainwane/zulip,eeshangarg/zulip,brockwhittaker/zulip,kou/zulip,mahim97/zulip,tommyip/zulip,tommyip/zulip,ryanbackman/zulip,eeshangarg/zulip,christi3k/zulip,dhcrzf/zulip,rishig/zulip,timabbott/zulip,ryanbackman/zulip,jackrzhang/zulip,rishig/zulip,j831/zulip,jackrzhang/zulip,andersk/zulip,verma-varsha/zulip,j831/zulip,zulip/zulip,hackerkid/zulip,verma-varsha/zulip,vabs22/zulip,hackerkid/zulip,jphilipsen05/zulip,brainwane/zulip,jackrzhang/zulip,brainwane/zulip,jrowan/zulip,Galexrt/zulip,j831/zulip,vaidap/zulip,vaidap/zulip,timabbott/zulip,zulip/zulip,timabbott/zulip,jphilipsen05/zulip,jackrzhang/zulip,jphilipsen05/zulip,rishig/zulip,hackerkid/zulip,ryanbackman/zulip,shubhamdhama/zulip,amanharitsh123/zulip,rishig/zulip,ryanbackman/zulip,kou/zulip,Galexrt/zulip,rht/zulip,jackrzhang/zulip,brainwane/zulip,hackerkid/zulip,verma-varsha/zulip,mahim97/zulip,andersk/zulip,jphilipsen05/zulip,rht/zulip,shubhamdhama/zulip,showell/zulip,synicalsyntax/zulip,Galexrt/zulip,jrowan/zulip,amanharitsh123/zulip,brainwane/zulip,andersk/zulip,vaidap/zulip,eeshangarg/zulip,mahim97/zulip,christi3k/zulip,vabs22/zulip,kou/zulip,zulip/zulip,punchagan/zulip,showell/zulip,vaidap/zulip,brainwane/zulip,dhcrzf/zulip,kou/zulip,punchagan/zulip,showell/zulip,andersk/zulip,christi3k/zulip,ryanbackman/zulip,ryanbackman/zulip,eeshangarg/zulip,synicalsyntax/zulip,rishig/zulip,showell/zulip,zulip/zulip,christi3k/zulip,verma-varsha/zulip,amanharitsh123/zulip,hackerkid/zulip,timabbott/zulip,rht/zulip,synicalsyntax/zulip,kou/zulip,amanharitsh123/zulip,shubhamdhama/zulip,j831/zulip,tommyip/zulip,timabbott/zulip,jrowan/zulip,Galexrt/zulip,jackrzhang/zulip,eeshangarg/zulip,punchagan/zulip,vaidap/zulip,dhcrzf/zulip,mahim97/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,synicalsyntax/zulip,punchagan/zulip,rishig/zulip,timabbott/zulip,vabs22/zulip,vaidap/zulip,shubhamdhama/zulip,tommyip/zulip,dhcrzf/zulip,showell/zulip,shubhamdhama/zulip,synicalsyntax/zulip,Galexrt/zulip,vabs22/zulip,punchagan/zulip,rht/zulip,jphilipsen05/zulip,Galexrt/zulip,verma-varsha/zulip,vabs22/zulip,christi3k/zulip,zulip/zulip