Create project_security_resource_counts_table

What does this MR do and why?

In this MR I am creating a table for persisting the current number of vulnerabilities a project has.
This is documented in this issue

I am creating:

• Table project_limits with the columns:
  • id
  • project_id (foreign key to projects)
  • namespace_id (foreign key to namespaces) - This is done in a separate migration step to avoid adding two foreign keys in the same step.
  • vulnerability_count

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