Split api_controller actions into their own controllers

[gravitystorm]

I've been planning on a refactoring of api-related controllers for a while. For further background reading, see gravitystorm#27.

I want to refactor the api-related controllers to inherit from a common controller (in the same way that regular controllers normally inherit from ApplicationController). The only convention that I found has this base controller named as "ApiController", but we're already using that name. So I thought about renaming our current ApiController, to free up the name for reuse, but I couldn't figure out a good descriptive name for the actions in the current ApiController. And then I realised that's because they are mostly unrelated to each other, and so I could split them into separate controllers instead.

So that's what this PR does - it splits the ApiController into separate controllers for each unrelated action.

I've chosen to put them in an "api" namespace (aka subdirectory) since I plan to do this for the rest of the api-related controllers. It is also the end-goal for other work that I'm doing on supporting multiple API versions (see gravitystorm#28), so creating the new controllers in the subdirectory should avoid further renaming.

[tomhughes]

One (genuine) test failure:

ExportControllerTest#test_finish_osm:
ActionController::UrlGenerationError: No route matches {:action=>"map", :bbox=>#<BoundingBox:0x0000556f20be2028 @min_lon=0.0, @min_lat=50.0, @max_lon=1.0, @max_lat=51.0>, :controller=>"api", :format=>"osm"}
    app/controllers/export_controller.rb:16:in `finish'
    test/controllers/export_controller_test.rb:20:in `test_finish_osm'


bin/rails test test/controllers/export_controller_test.rb:19

[gravitystorm]

Oops. I'd already fixed that when working on #2161 but had forgotten to update this branch after some rebasing. I've fixed this now with a force-push.