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/views/about/_version.html.haml

@@ -0,0 +1,4 @@
+.panel
+  .panel-header= t 'about.version'
+  .panel-body
+    %strong= version.version_number

app/views/about/more.html.haml

@@ -30,3 +30,4 @@
     .sidebar
       = render 'contact', contact: @instance_presenter
       = render 'links'
+      = render 'version', version: @instance_presenter

app/views/api/v1/instances/show.rabl

@@ -4,3 +4,4 @@ node(:uri)         { site_hostname }
 node(:title)       { Setting.site_title }
 node(:description) { Setting.site_description }
 node(:email)       { Setting.site_contact_email }
+node(:version)     { Mastodon::VERSION }