[wip] ERD diagrams

This requires a patch to work:

```diff
diff --git a/app/models/journal/base_journal.rb b/app/models/journal/base_journal.rb
index a44cd5a..7d77167 100644
--- a/app/models/journal/base_journal.rb
+++ b/app/models/journal/base_journal.rb
@@ -30,7 +30,7 @@
 class Journal::BaseJournal < ActiveRecord::Base
   self.abstract_class = true
 
-  belongs_to :journal
+  # belongs_to :journal
 
   def journaled_attributes
     attributes.symbolize_keys.select{|k,_| self.class.journaled_attributes.include? k}
```

To generate:

```sh
brew install graphviz
rake erd
```