Provide option to merge terms across vocabularies

Problem/Motivation

I have a set of terms that were grouped into one large catch-all vocabulary. Later on a set of views and filters were needed that dictate that these terms should be more structured into separate vocabularies. I would like to create the terms in the new vocabularies and then use the merge feature to update all content to point to the new vocabularies.

Steps to reproduce

Currently the "merge" tab only appears as a local task in a vocabulary

Proposed resolution

Update API and interface to allow cross-vocabulary merging.

Remaining tasks

Besides interface and API changes this will require some handling or messaging for fields that contain one or more source terms that don't support the target bundle.

User interface changes

Add a merge local task on the main vocabularies listing page.
Use something like autocomplete to find terms to merge. Allow multiple "source" terms to merge to one target.

API changes

TBD