refactor: Simplify user_last_seen_time_status().
refactor: Simplify user_last_seen_time_status().

We are about to stop supporting the presence status of "unknown."
Part of this fix is to stop checking for that status.

The implication of this change is that when we go
to display the time a user was last online, we now
mostly just look to see if presence.last_active_date
is undefined.  We were wary of that approach before, but it
is probably the most sane approach here.

I updated the comment abover this section to reflect
our philosophy going forward.

BTW the timestamp is kind of buried in the UI for now, as you have to
open the popover and then hover over the circular presence
indicator.

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