Support fallback to default paths

As per JIRA 4.2, custom index and attachment paths are not allowed anymore. If you're trying to import a backup file from < 4.2, JIRA will honour the custom paths and use them. If the directories don't exist, the importer asks you if you want to fallback to the default paths.

The Studio importer will report this type of error to the frontend, but does not offer the option to fallback. This causes extra effort for Support, since the current process, to just take the customer's file as it is and change the paths later, won't work anymore.

Current workaround: Open XML file, search for OSPropertyEntry elements with propertyKey jira.path.attachments.use.default.directory and jira.path.index.use.default.directory, copy the value of the id attribute and search for the corresponding OSPropertyNumber elements with that id. Change their value from 0 to 1.

The task here is to support this fallback for the Studio importer as well.