Fix "Creator" and "Contributor" attributes

[stranak]

See http://schema.datacite.org/meta/kernel-3/doc/DataCite-MetadataKernel_v3.0.pdf

• Creator has 1-n occurrences. All standard authors should be "Creator". For that we need hierarchical metadata and repeatable elements? #135 .
• Contributor is clearly reserved for secondary authors, like somebody that "also helped" in some smaller way.
• I.e. our current understanding as "First author=Creator" and "other authors=contributors" is wrong.

[stranak]

By the way, do we have other problems and inconsistencies with DataCite metadata scheme? Our design is built around the idea of requiring just DataCite required attributes and complying to the rest of their schema as much as possible too.

[peskk3am]

Currently there is an input field for a semicolon separated list of authors.
"add author" button with dynamically displayed new input field should be added.
(do we want this also for tags?)

[emanueldima]

I think we want to have this in general for many/most elements. The cardinality of an element should be marked in the metadata, and the UI should dynamically add fields based on this property.

[peskk3am]

Closed by #269