Miscellaneous cycle-analytics-related changes.

1. Add indexes to `CreateMergeRequestsClosingIssues` columns.
2. Remove an extraneous `check_if_open` check that is redundant now.

It would've been better to rebase this in, but that's not possible
because more people are working on this branch.

app/models/issue.rb

@@ -210,11 +210,7 @@ class Issue < ActiveRecord::Base
       note.all_references(current_user, extractor: ext)
     end
 
-    if check_if_open
     ext.merge_requests.select { |mr| mr.open? && mr.closes_issue?(self) }
-    else
-      ext.merge_requests.select { |mr| mr.closes_issue?(self) }
-    end
   end
 
   def moved?

db/migrate/20160915042921_create_merge_requests_closing_issues.rb

@@ -25,8 +25,8 @@ class CreateMergeRequestsClosingIssues < ActiveRecord::Migration
 
   def change
     create_table :merge_requests_closing_issues do |t|
-      t.references :merge_request, foreign_key: true, null: false
-      t.references :issue, foreign_key: true, null: false
+      t.references :merge_request, foreign_key: true, index: true, null: false
+      t.references :issue, foreign_key: true, index: true, null: false
 
       t.timestamps null: false
     end