[#6456] Refactored ProjectImporterDispatcher to Allura to remove dependency on FI

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6456] Added documentation for ForgeImporters package

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6456] Add project_update event to GC project and code importer

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>