Add HTTP proxy support.

Add support for using a proxy for HTTP requests. Only used for external
requests, such as downloading files or talking to source sites such as
Pixiv or Twitter, not for internal requests, such as talking to IQDB or
Reportbooru.

app/logical/twitter_api_client.rb

@@ -3,13 +3,13 @@
 class TwitterApiClient
   extend Memoist
 
-  attr_reader :api_key, :api_secret
+  attr_reader :api_key, :api_secret, :http
 
   # Create a Twitter API client
   # @param api_key [String] the Twitter API key
   # @param api_secret [String] the Twitter API secret
-  def initialize(api_key, api_secret)
-    @api_key, @api_secret = api_key, api_secret
+  def initialize(api_key, api_secret, http: Danbooru::Http.new)
+    @api_key, @api_secret, @http = api_key, api_secret, http
   end
 
   # Authenticate to Twitter with an API key and secret and receive a bearer token in response.
@@ -17,14 +17,13 @@ class TwitterApiClient
   # @return [String] the Twitter bearer token
   # @see https://developer.twitter.com/en/docs/authentication/api-reference/token
   def bearer_token(token_expiry = 24.hours)
-    http = Danbooru::Http.basic_auth(user: api_key, pass: api_secret)
-    response = http.cache(token_expiry).post("https://api.twitter.com/oauth2/token", form: { grant_type: :client_credentials })
+    response = http.basic_auth(user: api_key, pass: api_secret).cache(token_expiry).post("https://api.twitter.com/oauth2/token", form: { grant_type: :client_credentials })
     response.parse["access_token"]
   end
 
   # @return [Danbooru::Http] the HTTP client to connect to Twitter with
   def client
-    Danbooru::Http.auth("Bearer #{bearer_token}")
+    http.auth("Bearer #{bearer_token}")
   end
 
   # Fetch a tweet by id.