Ensure all HTML tags in issue descriptions are escaped / enclosed in code backticks

Example: https://github.com/helen/wp-themes-a11y-scanning/issues/12

Start by reproducing the behavior from the example issue at https://github.com/helen/wp-themes-a11y-scanning/issues/12, then trace the entry point that generates GitHub issue descriptions. Check the generated text for HTML tags and add coverage for the reported case; done means tags are escaped or enclosed in code backticks.

索引モデルが issue の本文から書いたものです。