http: split requests into internal and external requests.

Split requests made by Danbooru::Http into either internal or external
requests. Internal requests are API calls to internal services run by
Danbooru. External requests are requests to external websites, for
example fetching sources or downloading files. External requests may use
a HTTP proxy if one is configured. Internal requests don't.

Fixes a few source extractors not using the HTTP proxy for certain API calls.

app/logical/deviant_art_api_client.rb

@@ -21,10 +21,10 @@ class DeviantArtApiClient
   class Error < StandardError; end
   BASE_URL = "https://www.deviantart.com/api/v1/oauth2/"
 
-  attr_reader :client_id, :client_secret
+  attr_reader :client_id, :client_secret, :http
 
-  def initialize(client_id, client_secret)
-    @client_id, @client_secret = client_id, client_secret
+  def initialize(client_id, client_secret, http)
+    @client_id, @client_secret, @http = client_id, client_secret, http
   end
 
   # https://www.deviantart.com/developers/http/v1/20160316/deviation_single/bcc296bdf3b5e40636825a942a514816
@@ -54,7 +54,7 @@ class DeviantArtApiClient
     params = { access_token: access_token.token, **params }
 
     url = URI.join(BASE_URL, url).to_s
-    response = Danbooru::Http.cache(1.minute).get(url, params: params)
+    response = http.cache(1.minute).get(url, params: params)
     response.parse.with_indifferent_access
   end