Added individual settings fields to search keywords

[betschki]

The goal of this commit is to make all the individual setting fields searchable by adding them to the keywords. This should significantly improve UX when trying to look for "comments", for example, which one might not suspect to be paired with "Access" settings in memberships.

• There's a clear use-case for this code change, explained below
• Commit message has a short title & references relevant issues
• The build will pass (run yarn test:all and yarn lint)

[betschki]

This builds upon #18608 and is related to a discussion in the Ghost forum here: https://forum.ghost.org/t/searching-the-new-ui-bug/42043/2

[daniellockyer]

Thanks @betschki! 🙂