Refactor lang file, edit flash message copy

- removed duplicate lang labels
- moved borrower flash messages to lang directory
- moved labels common to lenders and borrowers to common directory (we should make the common directory lang files translatable)
- moved lender-only labels to lender file
- for lender labels, applied the organizing principle that only copy used in more than one place should go in the lang files (if we want to have unique copy in the lang files, we ought to apply that principle to the whole site)
- edited flash message copy