Desktop: Fixes #8530: Rich text editor: Use fewer  s in markdown while still preserving initial paragraph indentation

[personalizedrefrigerator]

Summary

This partially reverts a change made in #8468. This should fix #8530.

Before #8468, turndown replaced HTML nonbreaking spaces with their corresponding unicode characters. After, turndown replaced nonbreaking spaces with the HTML escape  .

While this fixes an issue where leading spaces would disappear from paragraph blocks when switching notes, it makes markdown much more difficult to read. For example,

    This is a test. 

instead of

    This is a test. 

(some of the above spaces are nonbreaking).

This pull request mostly reverts this change by only replacing the first nonbreaking space with an HTML escape sequence:

    This is a test. 

When rendered, this preserves the leading indentation while minimizing HTML-escaped  s.

(A leading HTML-escaped   seems to be necessary to preserve indentation).

Testing

This pull request does have an automated test. However, it can be tested manually. To do so,

1. Open the rich text editor.
2. Create a new paragraph with leading spaces (e.g. this is a test).
3. Switch to the markdown editor
4. Switch back to the rich text editor

In step 3, the markdown should only have a single   ­— the leading space. In step 4, the paragraph should still have leading spaces.

[personalizedrefrigerator]

replaceNonbreakingSpaces was not present in upstream turndown and was added by #8468. It is no longer used and has thus been removed.