Add continents and oceans to the filters on the article page

[nuest]

Use a gazetteer to identify the continent(s) or ocean(s) that encompasses a full or partial geometry, save this information with the article, and then expose that as a category filter in the right hand side bar for the full article list:

See OPTIMAP for datasets to handle the ocean geometries, possibly save simplified pre-processed geometries somewhere, in the Wiki or attached to a GitHub issue?

[nuest]

In terms of basic processing capabilities, this issue matches #33

[nuest]

needs a little "(i)" info icon next to the headling of the selection list that explains the implied concept of spatial coverage; the statistics from #33 are a prerequisite for this issue to have quick access to (cashed) numbers of works within a continent/ocean to display next to the region name;