FIX: Replace deprecated URI.encode, URI.escape, URI.unescape and URI.unencode (#8528)

The following methods have long been deprecated in ruby due to flaws in their implementation per http://blade.nagaokaut.ac.jp/cgi-bin/vframe.rb/ruby/ruby-core/29293?29179-31097:

URI.escape
URI.unescape
URI.encode
URI.unencode
escape/encode are just aliases for one another. This PR uses the Addressable gem to replace these methods with its own encode, unencode, and encode_component methods where appropriate.

I have put all references to Addressable::URI here into the UrlHelper to keep them corralled in one place to make changes to this implementation easier.

Addressable is now also an explicit gem dependency.

script/import_scripts/ipboard.rb

@@ -972,7 +972,7 @@ EOM
     User.find_each do |u|
       ucf = u.custom_fields
       if ucf && ucf["import_id"] && ucf["import_username"]
-        username = URI.escape(ucf["import_username"])
+        username = UrlHelper.encode_component(ucf["import_username"])
         Permalink.create(url: "#{USERDIR}/#{ucf['import_id']}-#{username}", external_url: "/users/#{u.username}") rescue nil
         print '.'
       end