mypy: Fix errors in new bugdown module.
mypy: Fix errors in new bugdown module.

I introduced these when making final changes before merging.

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