treewide: replace {-inbox} with {ibx} for consistency
{ibx} is shorter and is the most prevalent abbreviation
in indexing and IMAP code, and the `$ibx' local variable
is already prevalent throughout.
In general, the codebase favors removal of vowels in variable
and field names to denote non-references (because references are
"lighter" than non-references).
So update WWW and Filter users to use the same code since
it reduces confusion and may allow easier code sharing.