refactor: Rename bugdown_convert to markdown_convert.
refactor: Rename bugdown_convert to markdown_convert.

This commit is part of series of commits aimed at renaming bugdown to
markdown.

andersk/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,showell/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,brainwane/zulip,andersk/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,kou/zulip,brainwane/zulip,punchagan/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,brainwane/zulip,zulip/zulip,rht/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,andersk/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,showell/zulip,brainwane/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,zulip/zulip,synicalsyntax/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,synicalsyntax/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,kou/zulip