sources: add Source::URL class; factor out Source::URL::Twitter.

Introduce a Source::URL class for parsing URLs from source sites. Refactor the Twitter
source strategy to use it.

This is the first step towards factoring all the URL parsing logic out of source
strategies and moving it to subclasses of Source::URL. Each site will have a subclass
of Source::URL dedicated to parsing URLs from that site. Source strategies will use
these classes to extract information from URLs.

This is to simplify source strategies. Most sites have many different URL formats we have
to parse or rewrite, and handling all these different cases tends to make source
strategies very complex. Isolating the URL parsing logic from the site scraping logic
should make source strategies easier to maintain.

app/logical/danbooru/url.rb

@@ -4,29 +4,42 @@ module Danbooru
   class URL
     class Error < StandardError; end
 
+    # @return [String] The original URL as a string.
     attr_reader :original_url, :url
-    delegate_missing_to :url
 
-    # Parse a string into an URL, or raise an exception if the string is not a valid HTTPS or HTTPS URL.
+    # @return [Addressable:URI] The parsed and normalized URL.
+    attr_reader :url
+
+    delegate :domain, :host, :site, :path, to: :url
+
+    # Parse a string into a URL, or raise an exception if the string is not a valid HTTPS or HTTPS URL.
     #
-    # @param string [String]
-    # @return [Danbooru::URL]
-    def initialize(string)
-      @original_url = string
-      @url = Addressable::URI.heuristic_parse(string).display_uri
+    # @param url [String, Danbooru::URL]
+    def initialize(url)
+      @original_url = url.to_s
+      @url = Addressable::URI.heuristic_parse(original_url).display_uri
       @url.path = nil if @url.path == "/"
-      raise Error, "#{string} is not an http:// URL" if !@url.normalized_scheme.in?(["http", "https"])
+
+      raise Error, "#{original_url} is not an http:// URL" if !@url.normalized_scheme.in?(["http", "https"])
     rescue Addressable::URI::InvalidURIError => e
       raise Error, e
     end
 
-    # Parse a string into an URL, or return nil if the string is not a valid HTTP or HTTPS URL.
+    # Parse a string into a URL, or raise an exception if the string is not a valid HTTPS or HTTPS URL.
     #
-    # @param string [String]
+    # @param url [String, Danbooru::URL]
     # @return [Danbooru::URL]
-    def self.parse(string)
-      new(string)
-    rescue StandardError => e
+    def self.parse!(url)
+      new(url)
+    end
+
+    # Parse a string into a URL, or return nil if the string is not a valid HTTP or HTTPS URL.
+    #
+    # @param url [String, Danbooru::URL]
+    # @return [Danbooru::URL]
+    def self.parse(url)
+      parse!(url)
+    rescue Error
       nil
     end
 
@@ -42,7 +55,7 @@ module Danbooru
 
     # @return [Hash] the URL's query parameters
     def params
-      url.query_values.with_indifferent_access
+      url.query_values.to_h.with_indifferent_access
     end
   end
 end