Remove Darkfish generator and make Aliki standalone

Darkfish has been deprecated since Aliki became the default theme.
This makes Aliki a standalone generator by inlining all inherited
methods from Darkfish, then removes Darkfish and JsonIndex entirely.

- Make RDoc::Generator::Aliki a standalone class (no longer inherits
  from Darkfish), with all template rendering infrastructure inlined
- Remove RDoc::Generator::Darkfish and RDoc::Generator::JsonIndex
- Remove darkfish and json_index template directories
- Remove darkfish and json_index test files
- Update RDoc::Servlet to use Aliki instead of Darkfish
- Rewrite servlet search endpoints to use Aliki's search index format
- Remove autoload entries and require for darkfish
- Update tests to reflect Aliki as the sole HTML generator

Author: st0012

lib/rdoc/generator.rb

@@ -2,7 +2,7 @@
 ##
 # RDoc uses generators to turn parsed source code in the form of an
 # RDoc::CodeObject tree into some form of output.  RDoc comes with the HTML
-# generator RDoc::Generator::Darkfish and an ri data generator
+# generator RDoc::Generator::Aliki and an ri data generator
 # RDoc::Generator::RI.
 #
 # == Registering a Generator
@@ -44,8 +44,6 @@ module RDoc::Generator
   autoload :Markup,   "#{__dir__}/generator/markup"
 
   autoload :Aliki,     "#{__dir__}/generator/aliki"
-  autoload :Darkfish,  "#{__dir__}/generator/darkfish"
-  autoload :JsonIndex, "#{__dir__}/generator/json_index"
   autoload :RI,        "#{__dir__}/generator/ri"
   autoload :POT,       "#{__dir__}/generator/pot"