Ability to fork a public tag of another user

[tanius]

The ability to "fork" a tag of another user is the basic feature to enable collaboration between ethnographers in this software. Obviously, forking should only be possible for public tags.

When forking a tag, this is what should happen:

1. Create a new term in Drupal's openethnographer vocabulary, using the same name as the original tag, but a different author and a different tid ID.
2. In the new tag's taxonomy term, record the tid of the tag it was forked from. Can be done by adding a term reference field to the openethnographer vocabulary.
3. Duplicate annotation table records, changing the forked tag's tid to the new tag's tid in the duplicate. This redundancy is necessary to allow the fork's owner to delete annotations created with the original tag.
4. In the Annotator's tagging auto-complete form, only suggest tags authored by the annotating user. Including forked tags, but not including others' public tags that the user did not fork yet.

For the context: later (after version 1.0), there will be another feature for merging tagging changes of ones "origin" tag and other tags in the same fork network.