DOI + ref-text lookup

Adds a tool (in the **Metadata** tab) that looks up DOIs and reference text via the CrossRef API. The user sees a list of ranked results, and can choose to update the current study's DOI and/or reference text based on any result.

The search is done using the reference text already entered. If this field is empty, we send them to http://search.crossref.org/ in a new browser window.

**N.B.** If the curator uses the chosen reference text, I'm reducing it to plaintext since we haven't really planned for markup in this field.