Plaintext mails improperly use HTML entities

Noted this in issue gitlab-ce#26463 but might have not been noticed since it was resolved. Spotted it again today which reminded me. When you disable HTML mails, it still uses HTML entities for certain characters. For instance:

oops, can&# 39;t reference before assignment

(space added for clarity and so it isn't rendered). This should be an apostrophe not "&# 39;".