refactor: Rename bugdown to backend markdown in markdown.js.
refactor: Rename bugdown to backend markdown in markdown.js.

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

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