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/source/extractor/deviant_art.rb

@@ -189,10 +189,7 @@ module Source
       memoize :uuid
 
       def api_client
-        api_client = DeviantArtApiClient.new(
-          Danbooru.config.deviantart_client_id,
-          Danbooru.config.deviantart_client_secret
-        )
+        api_client = DeviantArtApiClient.new(Danbooru.config.deviantart_client_id, Danbooru.config.deviantart_client_secret, http)
         api_client.access_token = Cache.get("da-access-token", 11.weeks) do
           api_client.access_token.to_hash
         end

app/logical/source/extractor/fantia.rb

@@ -157,7 +157,7 @@ class Source::Extractor
     end
 
     def http
-      Danbooru::Http.new.cookies(_session_id: Danbooru.config.fantia_session_id)
+      super.cookies(_session_id: Danbooru.config.fantia_session_id)
     end
   end
 end

app/logical/source/extractor/furaffinity.rb

@@ -67,7 +67,7 @@ class Source::Extractor
     end
 
     def http
-      Danbooru::Http.new.cookies(a: Danbooru.config.furaffinity_cookie_a, b: Danbooru.config.furaffinity_cookie_b, sfw: 0)
+      super.cookies(a: Danbooru.config.furaffinity_cookie_a, b: Danbooru.config.furaffinity_cookie_b, sfw: 0)
     end
   end
 end

app/logical/source/extractor/twitter.rb

@@ -130,7 +130,7 @@ class Source::Extractor
     end
 
     def api_client
-      TwitterApiClient.new(Danbooru.config.twitter_api_key, Danbooru.config.twitter_api_secret)
+      TwitterApiClient.new(Danbooru.config.twitter_api_key, Danbooru.config.twitter_api_secret, http: http)
     end
 
     def api_response