per-language notifications and new original string notifications

[msykes]

There's a few main changes:

• Added per-language email notification settings for user to choose from. They can now subscribe to a specific language, or a whole project. Each language subscription will receive its own email, whereas projects get their own email with all languages with waiting translations.
• Changed notifications to be sent daily, weekly or monthly, meaning a user won't be notified every day for the same string awaiting translation if it isn't translated. If daily for example, an email will be sent each day summing up new waiting strings from that day, or no email is sent at all.
• Also added inclusion of notification for new originals needing if a user subscribes to a language or project