refactor: Rename bugdown to markdown in logging_handlers.py.
refactor: Rename bugdown to markdown in logging_handlers.py.

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

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