Add Gantt support

(this is not a duplicate, don't close)

Scrum does not have a prescription for neither road map development/oversight nor proper dependency management (between stories) for large projects, which would be grouped (in scrum speak) by epics. This shortfall in the methodology usually is solved by managing external, 10k foot, views on what needs to be done. This is done in road map documents, ms project, etc.

The only other agile methodology officially supported by jira now is Kanban, which is geared towards work similar to support and other operational-like tasks that are flowing in. Work managed by Kanban is not meant to be planned far ahead.

This Improvement request is meant to address necessary gap in jira's functionality and is a core feature that is necessity for any project substantial in size. This is not a request for additional plugin nor request for adding support for already existing one (for an On Demand setup).

1. Add a capability to import MS Project project.
2. Add a capability to export as MS Project project.
3. Add a capability to sync with MS Project project.
4. Add a capability to visualize all (or filtered) stories/epics as Gantt chart
5. In Gantt chart story/epic view, add Predecessors capability with support for finish-to-start, finish-to-finish, start-to-start and start-to-finish, with lead and lag time offsets.