Closes#13484.
These options tell zulip whether to prefer the plaintext or html version
of the email message. prefer-text is the default behavior, so including
the option doesn't change anything as of now, but we're adding it to
prepare to potentially change the default behavior in the future.
Note we're no longer using subscriptions_html in the help docs, so no need
to test for it. There is already a test for subscriptions_html in
IntegrationTest.
Mateusz Mandera
Rishi Gupta
amanagr
Tim Abbott
Cynthia Lin