[models] project.external_tracker_pattern
- add an external_tracker_pattern attribute for ability to reference bugs in outside tracker
- add matching migration
- update views accordingly
[lib] - markdown update for external trackers
- update the issue matching pattern to handle references such as bug xxx, issue yyy, as well as the original #xxx
- introduce an optional <text> regex variable, allowing back references to the original string when generating links to issues
- use @project.external_tracker_pattern when available
[views] admin/projects
- show external tracker pattern if defined