Allow the user to manually download nested relations

[froestel]

As said in #623 it will cause huge amounts of data when downloading all nested relations by default. But there are/will be relations which can't be accessed otherwise because their members are only relations which again only contain relations. Like a planet-relation containing continent-relations which in turn contain country-relations which contain a capital-relation that then finally contains a node. When downloading the BBox of that node now you'll only receive the capital- and country-relation, which makes it impossible to add another country to the continent-relation.

So a button allowing the user to make a /relation/$id/relations-request would be cool. Additionally a button to request all members of a relation (/relation/$id/full) would come in handy.

regards, Sven

[Gubaer]

Both requested actions are implemented in the relation editor sind r1806.