Extend comment email notification API

[voidxnull]

Server-side for #937

Notable changes:

• Refactored email templates to be more modular.
• Added daily and weekly tasks for delivering comment notification.
• Replaced the mute_all_posts user option with comment_notifications that can be set to: 'on', 'off', 'daily', 'weekly'.
• The updateUser controller was modified to accept the new option (more.comment_notifications).
• Added email previews (/api/v1/email-template/file_name.ejs).
• Refactored email tasks to use cached templates.
• Set up bunyan logging for tasks.

[coveralls]

Coverage decreased (-0.2%) to 59.348% when pulling d676b5070668a8ae17b165be552462e74b7ff6ec on feature/comment-notifications-api-ext into d9472273ee945d868028e73b5947b2968b319a2f on stable.