Fix authority link migration

[rosiel]

What does this Pull Request do?

The "Resource Type" terms were getting migrated along with their "external uri". However, some vocabularies (like Resource Types) don't have an "external uri" field but instead an "authority link" field. It was getting put in there - but that didn't make a complete viable field. This field has three (major) elements: uri, title, and source. All these three are visible in the GUI when editing the field and Source is mandatory. But when we migrate terms in, only the uri element is getting populated. So when we tried to display the term- on the node page or in views - it doesn't show up at all. (also maybe later fix the Authority Link field formatter... but for now we should make right with what we've got.)

This PR updates the migration so that

• the same URI in the uri is used as the title, since these are administrative terms and the URI needs to be legible to managers.
• The source field is populated. I added a column for that, and set all (except collection) to the DCMI terms. Collection (in Resource types) is the Bibo version. This keeps us from interfering with the Islandora Models.

Related GitHub Issue:

• [BUG] We're populating "Authority Link" fields (esp. in Resource Types terms) in a way that we cant use. #35

• Other Relevant Links: (Google Groups discussion, related pull requests,
  Release pull requests, etc.)

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[rosiel]

Closing; superseded by #39