Move IssueUser code to separate file (#836)

Also add unit tests

models/fixtures/issue.yml

@@ -3,6 +3,7 @@
   repo_id: 1
   index: 1
   poster_id: 1
+  assignee_id: 1
   name: issue1
   content: content1
   is_closed: false

models/fixtures/issue_user.yml

@@ -0,0 +1,23 @@
+-
+  id: 1
+  uid: 1
+  issue_id: 1
+  is_read: true
+  is_assigned: true
+  is_mentioned: false
+
+-
+  id: 2
+  uid: 2
+  issue_id: 1
+  is_read: true
+  is_assigned: false
+  is_mentioned: false
+
+-
+  id: 3
+  uid: 4
+  issue_id: 1
+  is_read: false
+  is_assigned: false
+  is_mentioned: false