Categories should be able to merge by renaming one to another

[rickybanister]

This has been completed at the project level for a specific project. The ability to merge categories by renaming should be moved into the trunk.

[rickybanister]

We should add an Apostrophe modal confirmation message.

Before executing a merge it should check and ask:

"The tag "blah" already exists. Do you want to merge these tags?
(Merge Tags) (Cancel)

[rickybanister]

This should also be accounted for in the Tags admin.

[geoffd]

Dan, please update this ticket and close if it is done.

[dordille]

Both categories and tags have merging implemented now. Merging happens automatically when you rename a category or tag to an already existing one. All objects linked to these objects are merged together.

[tboutell]

This got built but it looks like a listener was never added to the blog plugin. I'm doing that now.

[tboutell]

This works now in [3920] [3921] [3922]

Dan created a great infrastructure for this but it never made it to the blog plugin. I also extended it to the people plugin since MO needs that.

All plugins that add tables that are related to categories must listen to the a.merge_category event. For an easy way to respond to it, look at the apostrophePeoplePluginConfiguration and PluginaPersonToACategoryTable classes. Note that this code correctly handles tricky situations such as merging A to B where both categories already have the person in question.