apps: Update link to point to future Zulip iOS app ID.
apps: Update link to point to future Zulip iOS app ID.

Previously, this was linked to the now-removed old Dropbox iOS app for
Zulip.  The link won't work for a few more days, but we're just
waiting on app store approval and know the ID, at least.

Fixes #3267.

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