Bug 873477 - [SMS][MMS] Group participants string formatting update per v8.0

I believe this is incomplete and need someone with more i18n experience to review

- Renames `contact-title-text` => `others` to match other locales
- Updates en-US locale with correct displayed text

Other locales aren't updated because I'm not sure it's a 1-to-1 change



Signed-off-by: Rick Waldron <waldron.rick@gmail.com>