Add "Periodization" metadata field to archaeological metadata scheme

[lbownik]

We would like to add another field to our archaeological metadata scheme. This time it should allow user to select a period from standard Periodo gazetter.

  We would like store somewhere a copy of the whole Periodo database. Maybe the way we currently store the ROR’s will do? Periodo in JSON format is available here: https://perio.do/en/


    
  We would like to add a single metadata field to the archaeological scheme, in accordance with the mockup below. 

  We should add fancy mode of selecting the right period identifier from the list. We may use similar mechanism that we use for ROR fields: user should eventually provide an identifier for a period, but may also enter into the field some contextual information that would help him with making a proper selection. On this basis, having the copy of the database locally stored as in (1), we may prompt him with the possible periods to select. After the period is selected, the input is automagically filled in with the period identifier.

4.Having the identifier, we should show on adding/editing metadata page some other information regarding the selected periods, as in (2). Thanks to that, users will be able to identify the period they selected and fix possible mistakes (it would not be possible based on the identifier itself, as it does not convey any semantic information).