message scrolling: Add visible_bottom to info.
message scrolling: Add visible_bottom to info.

It's convenient to have visible_bottom as well
as top/height, and the extra computation is
trivial (it's just arithmetic, no extra jQuery
involved).

There's some minor cleanup here too.

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