tools: Fix message-screenshot.js failing to take screenshot.
tools: Fix message-screenshot.js failing to take screenshot.

To take screenshots, we log in as Desdemonia who doesn't seem
to have permissions to the streams where we capture screenshots.
This commit fixes it by logging in as Iago.

Also added a waitForNavigation until DOMContentLoaded to
avoid flakes that could occur because of us going to another URL
before Iago gets logged in.

showell/zulip,kou/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,shubhamdhama/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,kou/zulip,synicalsyntax/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,showell/zulip,showell/zulip,rht/zulip,kou/zulip,timabbott/zulip,brainwane/zulip,brainwane/zulip,timabbott/zulip,brainwane/zulip,synicalsyntax/zulip,andersk/zulip,showell/zulip,timabbott/zulip,synicalsyntax/zulip,andersk/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,timabbott/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,shubhamdhama/zulip,andersk/zulip,synicalsyntax/zulip,shubhamdhama/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,zulip/zulip,zulip/zulip,showell/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,timabbott/zulip,kou/zulip,timabbott/zulip