expanding scope of MPCite to include user-submitted DOIs

[dwinston]

@mkhorton, @tschaume and I are starting this. This will allow users to submit DOIs for resources related to a material -- keyed by material ID or matched to one via a provided CIF. These submissions will appear flagged until approved by MP staff for appropriateness. We will use CrossRef, etc. to generate proper BibTeX entries given the DOIs.