add per-account quota for total message size disk usage

so a single user cannot fill up the disk.
by default, there is (still) no limit. a default can be set in the config file
for all accounts, and a per-account max size can be set that would override any
global setting.

this does not take into account disk usage of the index database. and also not
of any file system overhead.

config/doc.go

@@ -588,6 +588,14 @@ describe-static" and "mox config describe-domains":
 	# (optional)
 	OutgoingTLSReportsForAllSuccess: false
 
+	# Default maximum total message size for accounts, only applicable if greater than
+	# zero. Can be overridden per account. Attempting to add new messages beyond the
+	# maximum size will result in an error. Useful to prevent a single account from
+	# filling storage. The quota only applies to the email message files, not to any
+	# file system overhead and also not the message index database file (account for
+	# approximately 15% overhead). (optional)
+	QuotaMessageSize: 0
+
 # domains.conf
 
 	# NOTE: This config file is in 'sconf' format. Indent with tabs. Comments must be
@@ -849,6 +857,13 @@ describe-static" and "mox config describe-domains":
 				# How long unique values are accepted after generating, e.g. 12h.
 				Period: 0s
 
+			# Default maximum total message size for the account, overriding any globally
+			# configured maximum size if non-zero. A negative value can be used to have no
+			# limit in case there is a limit by default. Attempting to add new messages beyond
+			# the maximum size will result in an error. Useful to prevent a single account
+			# from filling storage. (optional)
+			QuotaMessageSize: 0
+
 			# Mail that looks like spam will be rejected, but a copy can be stored temporarily
 			# in a mailbox, e.g. Rejects. If mail isn't coming in when you expect, you can
 			# look there. The mail still isn't accepted, so the remote mail server may retry