MappingBrowser: support database selection for occurrences

[nichtich]

• [ ] show database of each occurence (is included in field database), for instance as hover at the count.
• [ ] show selected database (http://uri.gbv.de/database/opac-gvk by default)
• [ ] send database URI with parameter database. Example query: http://coli-conc.gbv.de/occurrences/api?members=http://uri.gbv.de/terminology/bk/77.53&database=http://uri.gbv.de/database/opac-de-7
• [ ] select database via http://uri.gbv.de/database/api/dbkey?id={suggest} (see http://uri.gbv.de/database/ search box right above).

[stefandesu]

Needs changes in occurrences-api.

[stefandesu]

Should be taken into account for https://github.com/gbv/occurrences-server.

[nichtich]

Just showing the current name is enough: #711