Merge branch 'fix-migration-introduced-by-5387' into 'master'

Fix migration to make it fast

## What does this MR do?

Changing the migration introduced in !5387 so that it's fast. The idea is that the content of the `projects.has_external_wiki` is currently wrong, so we can just drop and recreate the column.

## Does this MR meet the acceptance criteria?

- [x] No CHANGELOG needed
- Tests
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5394

db/migrate/20160721081015_nullify_has_external_wiki_in_projects.rb → db/migrate/20160721081015_drop_and_readd_has_external_wiki_in_projects.rb

-class NullifyHasExternalWikiInProjects < ActiveRecord::Migration
+class DropAndReaddHasExternalWikiInProjects < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
   def up
-    execute("UPDATE projects SET has_external_wiki = NULL")
+    remove_column :projects, :has_external_wiki, :boolean
+    add_column :projects, :has_external_wiki, :boolean
   end
 
   def down