js: Replace ZulipDesktop useragent check for old desktop app.
js: Replace ZulipDesktop useragent check for old desktop app.

Checking for window.bridge !== undefined is how the old desktop app
had always been tested for in the JS codebase.

Also, "Zulip Desktop" is how this should have been spelled in the first place.

Tweaked by tabbott to provide a proper commit message.

Fixes #6580.

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