Add ability to vote for issues (BB-3647)

The site/master issue tracker is full of "+1" posts. It would make more sense to let people vote for issues (similar to Google code).

There may also be an argument for incorporating a "heat" metric similar to launchpad. Heat should be based on issue votes and priority if implemented. The simplicity of a vote mechanism may be prefferable, however.

Either way, a numerical value of the "score" which can be filtered and sorted in the main issue list views would make it easier to prioritise and plan development work for larger projects based on their perceived importance.