Allow branch merging to be executed as tasks rather than Plan Configuration/Branches policy

Currently, the Branch Updater and Gatekeepr merging capabilities are hard wired to the Plan Configuration/Branches page. These merging mechanisms should be available as tasks under the plan jobs section.

This would allow a build job to:

1. control when merging occurs
2. perform possibly multiple merges
3. initiate merging as a manual task after other tasks have completed
4. execute error response tasks on merge failure.

In short, I need more control over merging than the hard-wired mechanisms. I need the ability to implement change flow between branches and between repositories and designated places within the software construction process. I like CI merging however the current bamboo merge mechanisms are too limiting.