store class names instead of classes in acts_as_ registries

Storing classes in development may lead to problems when reloading
classes, as new versions of the class will be appended to the list, but
`instance` method will still find the old versions of the classes.

I considered adding caching for production environment, but additional
code is not justified by 3 times faster execution.

```ruby
require "benchmark/ips"

h = {"Project" => Project}

Benchmark.ips do |x|
  x.report("constantize") { "Project".constantize }

  x.report("hash access") { h["Project"] }

  x.compare!
end
```

```
Warming up --------------------------------------
         constantize     1.163M i/100ms
         hash access     3.397M i/100ms
Calculating -------------------------------------
         constantize     11.842M (± 1.5%) i/s -     59.291M in
5.007752s
         hash access     34.379M (± 1.3%) i/s -    173.239M in
5.039989s

Comparison:
         hash access: 34378546.0 i/s
         constantize: 11842325.0 i/s - 2.90x  slower
```

lib_static/open_project/acts/registry_methods.rb

@@ -30,7 +30,9 @@ module OpenProject
   module Acts
     module RegistryMethods
       def instance(model_name)
-        models.detect { |cls| cls.name == model_name.singularize.camelize }
+        class_name = model_name.singularize.camelize
+
+        class_name.constantize if class_names.include?(class_name)
       end
 
       def add(*models)
@@ -42,14 +44,14 @@ def add(*models)
             raise ArgumentError.new("Model #{model} does not include #{acts_as_method_name}")
           end
 
-          self.models << model
+          class_names << model.name
         end
       end
 
       private
 
-      def models
-        @models ||= Set.new
+      def class_names
+        @class_names ||= Set.new
       end
     end
   end