Fix some HTML problems

Summary: I'm too lazy to attaching them for diffs where they were introduced.

Test Plan:
/
/D1, wrote comment with code snippet
DarkConsole
commit detail, wrote comment
task detail, wrote comment

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4911