Allow RhodeCode maintainers to specify a custom bug tracker. This allows people who maintain large RhodeCode installations to setup their own bug tracker and respond to requests against their specific installation. The maintainer is then free to forward problems with RhodeCode to the canonical issue tracker on bitbucket. If the config option "bugtracker" is present, its value will be used with the "Report a bug" button. If left blank, this disables the button. If no value is present, then the default is used. This is so that the new config option doesn't break installations of RhodeCode upgrading to a newer version and to allow easier installation for the common use case.