typeahead: Fix typeahead positioning for iOS keyboard case.
typeahead: Fix typeahead positioning for iOS keyboard case.

When the iOS keyboard is open and up, the positioning gotten by
getBoundingClientRect will display a `top` value that is short by the
height of the keyboard, which will usually end up placing things north
of the top of the screen.

By changing to jQuery $.fn.offset instead, the positioning appears to
be correct in all cases; iOS keyboard up, down, and desktop usage.

Fixes: #6366.

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