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.

app/presenters/instance_presenter.rb

@@ -25,4 +25,8 @@ class InstancePresenter
   def domain_count
     Rails.cache.fetch('distinct_domain_count') { Account.distinct.count(:domain) }
   end
+
+  def version_number
+    Mastodon::VERSION
+  end
 end