Track CI minutes on a monthly basis at project level

What does this MR do?

Related to #277424 (closed)

This MR builds on top of !52915 (merged) where we track CI minutes on a monthly basis at namespace level.

This new tracking will eventually replace project_statistics.shared_runners_seconds since it provides also historical usage while the existing shared_runners_seconds is reset to 0 every month.

Database migrations

➜ rails db:migrate:redo VERSION=20210205084357

== 20210205084357 CreateCiProjectMonthlyUsage: reverting ======================
-- drop_table(:ci_project_monthly_usages)
   -> 0.0029s
== 20210205084357 CreateCiProjectMonthlyUsage: reverted (0.0106s) =============

== 20210205084357 CreateCiProjectMonthlyUsage: migrating ======================
-- create_table(:ci_project_monthly_usages, {:if_not_exists=>true})
   -> 0.0181s
== 20210205084357 CreateCiProjectMonthlyUsage: migrated (0.0207s) =============

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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