Adds version to about/more and API (#2181)

* Adds version.

* Cleans up code.

* Removes standalone endpoint and adds version to instance endpoint.

* Addresses feedback from #2181.

config/application.rb

@@ -9,6 +9,7 @@ Bundler.require(*Rails.groups)
 require_relative '../app/lib/exceptions'
 require_relative '../lib/paperclip/gif_transcoder'
 require_relative '../lib/paperclip/video_transcoder'
+require_relative '../lib/mastodon/version'
 
 Dotenv::Railtie.load