emoji_picker: Fix issue with emoji tab shifting.
emoji_picker: Fix issue with emoji tab shifting.

In this commit we basically do these things:
* Clear up section_head_offsets before pushing stuff in it so that
its size doesn't keep on growing indefinitely with time and users
opening emoji picker.
* Make use of popover element to find the correct element in DOM
to scan for section elements. This prevents us from filling stuff
twice into section_head_offsets because of presence of two
elements for '.emoji-popover-subheading' in DOM since popover
destroy is an async call.
* Using this popover element also helps in avoiding manuplation
of the DOM elements of the popover that was destroyed (Because
popover destroy is async it still maybe around). One instance of
this is associating scroll event with the right instance of
'.emoji-popover-emoji-map'.

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