Ensure <li>s are wrapped in a <ol> or <ul> after sanitizing HTML

[diox]

Fixes mozilla/addons#14989

Context

This matters when displaying the versions page, where each version is in a <li> already, and developers can have release notes containing <li> of their own. This can be seen:

• On dev: https://addons-dev.allizom.org/en-US/firefox/addon/ratopuse-weasp-vulturkal/versions/
• On prod: https://addons.mozilla.org/en-US/firefox/addon/bitwarden-password-manager/versions/

(Look at the output carefully, the HTML from the footer is duplicated, the "clone" appearing either in the sidebar or on top of the existing footer)

Testing

You have to manually craft HTML with <li> and no parent <ul> and get it passed to our sanitize functions to test this. That is harder to achieve now that we have markdown, so best to modify the entry in the database, or pick one that was already broken, or test with https://localhost:3000/en-US/firefox/addon/ratopuse-weasp-vulturkal/versions/ with the patch applied and unapplied to see the difference. Or look at the unit tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.29%. Comparing base (8173acd) to head (dc0c709).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #13544   +/-   ##
=======================================
  Coverage   98.28%   98.29%           
=======================================
  Files         267      268    +1     
  Lines       10630    10644   +14     
  Branches     3247     3255    +8     
=======================================
+ Hits        10448    10462   +14     
  Misses        169      169           
  Partials       13       13           

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