Introduced possibility to use sending user's locale when generating i18n links to big attachments.

The property com.openexchange.mail.transport.externalRecipients Locale in file 'transport.properties' respects special identifier user-defined which indicates to use that locale from the user who sends the email. This is the new default value, too.

# Set the identifier of the locale to use when composing text sent to external recipients; e.g en, or de
# The special identifier user-defined indicates to use that locale from the user who sends the email.
com.openexchange.mail.transport.externalRecipients Locale=user-defined