dkim 2048 bits - migration and zone file generation changes

* Add a migration to delete any existing DKIM key so that existing machines get a fresh 2048-bit key. (Sadly we don't support key rotation so the change is immediate.)
* Because the DNS record for a 2048-bit key is so much longer, the way we read OpenDKIM's DNS record text file had to be modified to combine an arbitrary number of TXT record quoted ("...") strings.
* When writing out the TXT record value, the string must be split into quoted ("...") strings with a maximum length of 255 bytes each, per the DNS spec.
* Added a changelog entry.

setup/migrate.py

@@ -95,6 +95,11 @@ def migration_7(env):
 	# Save.
 	conn.commit()
 
+def migration_8(env):
+	# Delete DKIM keys. We had generated 1024-bit DKIM keys.
+	# By deleting the key file we'll automatically generate
+	# a new key, which will be 2048 bits.
+	os.unlink(os.path.join(env['STORAGE_ROOT'], 'mail/dkim/mail.private'))
 
 def get_current_migration():
 	ver = 0