Allow user to import existing metadata via CSV

[kjantin]

If a user has a collection of documents, and they have already identified metadata/properties for these documents, can we develop a way for them to import this existing data from a CSV file?

This issue evolved from #843

[txau]

@kjantin #843 was a small issue describing a very precise functionality, small and well defined. This is a broader issue describing a very big functionality that may not happen any time soon. I think we should reopen the other one and keep this one more as a nice to have.

[bertver]

This afternoon I had a talk with a client who would benefit from the import of metadata. They have field offices in areas with low Internet connectivity which would enter data on Excel sheets which then would have to be regularly imported into a database.

[danicatalan]

@bertver Do they want to upload new data on each import or also update current documents and entities?

[bertver]

@danicatalan it is for uploading new data

[RafaPolit]

Solved in #2340 and #2347. Further improvements being discussed in #2310