Add Danbooru::URL class.

Introduce a Danbooru::URL class for dealing with URLs. This is a wrapper
around Addressable::URI that adds some additional helper methods. Most
significantly, the `parse` method only allows valid http/https URLs, and
it returns nil instead of raising an exception when the URL is invalid.

app/logical/danbooru/url.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module Danbooru
+  class URL
+    class Error < StandardError; end
+
+    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.
+    #
+    # @param string [String]
+    # @return [Danbooru::URL]
+    def initialize(string)
+      @original_url = string
+      @url = Addressable::URI.heuristic_parse(string).display_uri
+      @url.path = nil if @url.path == "/"
+      raise Error, "#{string} 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.
+    #
+    # @param string [String]
+    # @return [Danbooru::URL]
+    def self.parse(string)
+      new(string)
+    rescue StandardError => e
+      nil
+    end
+
+    # @return [String] the URL in normalized form
+    def to_s
+      url.to_str
+    end
+
+    # @return [Array<String>] the URL's path split into segments
+    def path_segments
+      path.split("/").compact_blank
+    end
+
+    # @return [Hash] the URL's query parameters
+    def params
+      url.query_values.with_indifferent_access
+    end
+  end
+end