css: Specify message_content line-height in relative units.
css: Specify message_content line-height in relative units.

This fixes (or at least reduces) a problem with inline `code blocks`
causing extra uneven vertical space below their line.  The absolute
line-height was being measured relative to the lower midline of the
smaller font-size in the code blocks.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

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