Support for multiple bug trackers

[Hlipa]

Currently, opengrok allows to configure only one bug tracking system (and URL pattern) which is same for all hosted projects. Problem is if the server hosted several projects which some of them uses different bug tracking system. Then the bug links are wrong.

It would be good to allow configure more URL patterns which could be assign to the project or group of projects.

[vladak]

The review pattern has the same limitation.