refactor: Introduce user_key variable for presence.
refactor: Introduce user_key variable for presence.

This is a first step toward moving toward user_ids
for clients that ask for slim presence payloads.

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