Broken linkification from inconsistent Markdown processing

[gibson042]

Parsing of ```…``` "fence" blocks and `…` inline code in ReplaceQuotedSubstrings does not match that of shared/common-adapters/markdown/shared.tsx, and as a result links are incorrectly detected.

For example:

• ``code` renders as «``code</strong>», while ``` ``https://example.com/ ``` renders as «``$>kb$…$<kb$`» with a preview box.

• ```\```fenced``` renders as

  \```fenced
  

  while ```\```https://example.com/``` renders as

  ` ``\ ` ``$>kb$…$<kb$`

  with a preview box.

• ```code``````https://example.com/``` renders as

  code```
  

  https://example.com/ ` ``

  (in which https://example.com/ is not a link)