Add association for Comments and RequestForComments

For the submission and comments, I mainly decided to use a `has_one` association. Based on the database schema, multiple request for comments could be allowed (i.e., for each file or submission), but this won't happen practically (since we always create new submissions and files). Hence, the `has_one` association is representing our relationship better.

app/models/code_ocean/file.rb

@@ -32,6 +32,7 @@ module CodeOcean
     has_many :files, class_name: 'CodeOcean::File'
     has_many :testruns
     has_many :comments
+    has_one :request_for_comment
     has_many :events_synchronized_editor, class_name: 'Event::SynchronizedEditor'
     alias descendants files