narrow: Extract update_narrow_title.
narrow: Extract update_narrow_title.

This just makes the flow of narrow.activate easier to follow.

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