Add index on marked_for_deletion_at in projects table

What does this MR do?

Add index on marked_for_deletion_at in projects table

It was pointed out here it is a good idea to add this index.

Screenshots

== 20191107220314 AddIndexToProjectsOnMarkedForDeletion: migrating ============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:projects, :marked_for_deletion_at, {:where=>"marked_for_deletion_at IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0124s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- add_index(:projects, :marked_for_deletion_at, {:where=>"marked_for_deletion_at IS NOT NULL", :algorithm=>:concurrently})
   -> 0.0078s
-- execute("RESET ALL")
   -> 0.0008s
== 20191107220314 AddIndexToProjectsOnMarkedForDeletion: migrated (0.0216s) ===

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #32935 (closed)