search: index byte size of a message for IMAP search
Searching for messages smaller than a certain size is allowed by
offlineimap(1), mbsync(1), and possibly other tools. Maybe
public-inbox-watch will support it, too.
I don't see a reason to expose searching by size via WWW search
right now (but maybe in the future, I could be convinced to).
Note: we only store the byte-size of the message in git,
this is typically LF-only and we won't have the correct
size after CRLF conversion for NNTP or IMAP.