Create copy for strings in the browser extension as well as webstore copy.
Messages not yet present in Extension:ReadingLists that are needed to support a reading list client:
- A success message (e.g., "Page added!")
- A reading lists-specific login required message (e.g., "You must be logged in to access your reading lists.")
- (optional) An introductory clause to precede an API error message (e.g., "Failed to add page:")