messageable_users: it's ok to message yourself

fixes #9829

users without any enrollments weren't included in
User.messageable_users, but messaging yourself should always be allowed.

Test plan:
  * log in as a user without any enrollments
    - Go to your inbox.  You should be able to message yourself by
      searching for your name.
    OR
    - Go to your profile page on an account with profiles enabled.  You
      should be able to see your profile.

Change-Id: If5182d807fe2f3150999d442d30202c22dffa4d1
Reviewed-on: https://gerrit.instructure.com/12819
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>

app/models/user.rb

@@ -2301,6 +2301,12 @@ class User < ActiveRecord::Base
         AND user_account_associations.user_id = users.id
         #{user_condition_sql}
     SQL
+    user_sql << <<-SQL unless options[:context]
+      SELECT #{MESSAGEABLE_USER_COLUMN_SQL}, NULL AS course_id, NULL AS group_id, NULL AS roles
+      FROM users
+      WHERE id = #{self.id}
+        #{user_condition_sql}
+    SQL
 
     if options[:ids]
       # provides a way for this user to start a conversation with someone

spec/models/user_spec.rb

@@ -791,6 +791,10 @@ describe User do
       @deleted_user.destroy
     end
 
+    it "should include yourself even when not enrolled in courses" do
+      @student.messageable_users(:ids => [@student.id]).should eql [@student]
+    end
+
     it "should only return users from the specified context and type" do
       set_up_course_with_users
       @course.enroll_user(@student, 'StudentEnrollment', :enrollment_state => 'active')