Support for new Tigris bug tracker in main website

Closes issue #1005 (depends on having issue #1006 in oh-bugimporters deployed!, see https://github.com/openhatch/oh-bugimporters/pull/57)

This patch adds support for the new Tigris bug tracker to the main website. I tested it in a local OpenHatch instance on my machine, and the "./run_importer.sh" script did its job fine. I hope this is sufficient as a basic test, if there is more required from my side just let me know.

I also added some basic documentation about how to develop and test new bug trackers, please feel free to correct my writing (not a native speaker here) or markup wherever it appears necessary.

Regards,

Dirk

I'm not sure if we need both these migrations, but that's okay.