message_edit: Change scroll calculations based on `.message_content`.
message_edit: Change scroll calculations based on `.message_content`.

This concerns the part where the message viewport is scrolled in
order to keep the message content and edit box textarea at the same
vertical position.

Earlier calculations involved use of `.message_top_line`. Similar
adjustments can be made using only the message_content (which also
makes the calculation simpler).

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