add spanish mode

note: when playing this: also strip any remaining user-facing instances of "publisher" instead of "admin"

Behavior

GIVEN a working signalboost number

• WHEN a new receiver joins with the message "HOLA"
• THEN the receiver will receive a welcome message in spanish
• AND THEN all subsequent messages will be sent to them in spanish

GIVEN an existing member alice on 'foo' whose language is set to "EN" (english)

• WHEN alice sends a message that says "ESPANOL", "ESPAÑOL", or "SPANISH"
• THEN alice will receive a confirmation response in spanish
• AND THEN all further command responses to alice will be in spanish

Implementation notes

• add spanish strings to messages.spanish.js (or maybe: messages.ES.js? i think the former is less confusing)
• (if not already present): add language field to subscriptions table
• (if not already implemented): refactor app/dispatcher/messenger to dispatch on subscription language when building string to send in message