We no longer include previous replies as "context", instead
we include and excerpt of the post being replied to at the bottom
of notifications, this information was previously missing.
Users may opt in to emailing previous replies if they wish
or opt out of "in-reply-to" which makes sense in some email clients that
are smarter about displaying a tree of replies.
- always means we always send previous replies with every email
- never means we do not
- "unless previously sent" ... is the default, in which we only email you each reply once
The default_email_previous_replies site setting can control this toggle
As it stands we load up user records quite frequently on the topic pages,
this in turn pulls all the columns for the users being selected, just to
discard them after they are loaded
New structure keeps all options in a discrete table, this is better organised
and allows us to easily add more column without worrying about bloating the
user table
