Add db column to indicate temporarily increased storage

What does this MR do?

This adds the column temporary_storage_increase_ends_on to namespace_limits and will be used to determine if a namespace is currently eligible for unlimited storage for that time or not. The period will be 30 days and can only be set once for a namespace. Therefore, when the temporary increase gets activated, we will set this field to Time.current + 30.days if it was not set before.

This MR only adds the db column, rest will follow in the epic Part of &3508 (closed).

FYI: If there are naming suggestions for that column, I am happy to hear them :)

bin/rails db:migrate
== 20200706170536 AddTemporaryStorageIncreaseToNamespaceLimits: migrating =====
-- add_column(:namespace_limits, :temporary_storage_increase_ends_on, :date, {:null=>true})
   -> 0.0016s
== 20200706170536 AddTemporaryStorageIncreaseToNamespaceLimits: migrated (0.0017s)

bin/rails db:rollback
== 20200706170536 AddTemporaryStorageIncreaseToNamespaceLimits: reverting =====
-- remove_column(:namespace_limits, :temporary_storage_increase_ends_on, :date, {:null=>true})
   -> 0.0008s
== 20200706170536 AddTemporaryStorageIncreaseToNamespaceLimits: reverted (0.0028s)

Screenshots

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

Closes #227054