emails: Fix duplicate <html> blocks in missed-message emails.
emails: Fix duplicate <html> blocks in missed-message emails.

Apparently, the missed-message templates have a slightly different
structure from our other email templates, which triggered a latent,
subtle bug in inline-email-css's effort to remove duplicate <html>
blocks from emails that had been generated by premailer.  Fix this
bug, and add appropriate assertions to prevent similar issues in the
future.

Fixes #11249.

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