css: Move markdown rendering to rendered_markdown class.
css: Move markdown rendering to rendered_markdown class.

This eliminates unnecessary use of the message_content CSS class in
favor of rendered_markdown, which makes more sense for places outside
messages where we display rendered Zulip markdown.

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