Allow Mantis importer to import into existing project

Up until JIRA 3.10, the Mantis importer appeared to able to import into an existing project. What it actually did was create a new project with the same key as the existing project (which shouldn't be possible) and import the issues into that project.

This was discovered and fixed in 3.10 (JRA-12827), but the 'fix' was to simply disallow importing into existing projects. Currently you'll get an error if you try:

Being able to import into an existing project is fairly essential, especially since, because of JRA-8248, one cannot bulk move projects around without losing versions/components.