config: rework http proxy options.

Replace the http_proxy_host, http_proxy_port, http_proxy_username, http_proxy_password options with
a single `Danbooru.config.http_proxy` option.

app/logical/danbooru/http.rb

@@ -131,11 +131,12 @@ module Danbooru
       use(cache: { expires_in: expires_in })
     end
 
-    def proxy(host: Danbooru.config.http_proxy_host, port: Danbooru.config.http_proxy_port.to_i, username: Danbooru.config.http_proxy_username, password: Danbooru.config.http_proxy_password)
-      return self if host.blank?
+    def proxy(url: Danbooru.config.http_proxy)
+      return self if url.blank?
+      parsed_url = Danbooru::URL.parse!(url)
 
       dup.tap do |o|
-        o.http = o.http.via(host, port, username, password)
+        o.http = o.http.via(parsed_url.host, parsed_url.port, parsed_url.http_user, parsed_url.password)
       end
     end
 

app/logical/danbooru/url.rb

@@ -34,7 +34,7 @@ module Danbooru
     # @return [Addressable:URI] The parsed and normalized URL.
     attr_reader :url
 
-    delegate :domain, :host, :site, :path, :query, to: :url
+    delegate :domain, :host, :port, :site, :path, :query, :password, to: :url
 
     # Parse a string into a URL, or raise an exception if the string is not a valid HTTP or HTTPS URL.
     #
@@ -111,6 +111,11 @@ module Danbooru
       parsed_domain.trd
     end
 
+    # @return [String, nil] The username in a `http://username:password@example.com` URL.
+    def http_user
+      url.user
+    end
+
     # @return [PublicSuffix::Domain]
     def parsed_domain
       @parsed_domain ||= PublicSuffix.parse(host)

config/danbooru_default_config.rb

@@ -578,16 +578,9 @@ module Danbooru
       "https://twitter.com/#{Danbooru.config.twitter_username}"
     end
 
-    def http_proxy_host
-    end
-
-    def http_proxy_port
-    end
-
-    def http_proxy_username
-    end
-
-    def http_proxy_password
+    # The proxy to use for outgoing HTTP requests.
+    def http_proxy
+      # "http://username:password@proxy.example.com:1080"
     end
 
     # The URL for the Reportbooru server (https://github.com/evazion/reportbooru).