Add Mastodon::Source.url (#4643)

* Add Mastodon::Source.url

* Update spec

* Refactor

Move things frmo Mastodon::Source to Mastodon::Version

lib/mastodon/version.rb

@@ -31,5 +31,22 @@ module Mastodon
     def to_s
       [to_a.join('.'), flags].join
     end
+
+    def source_base_url
+      'https://github.com/tootsuite/mastodon'
+    end
+
+    # specify git tag or commit hash here
+    def source_tag
+      nil
+    end
+
+    def source_url
+      if source_tag
+        "#{source_base_url}/tree/#{source_tag}"
+      else
+        source_base_url
+      end
+    end
   end
 end