Pass correct Time object to VSM DataCollector

What does this MR do?

Pass correct Time object to Cycle Analytics (Value Stream Management) DataCollector

This change makes sure that the underlying query works properly if Date parameters are given:

• The created_after parameter will be casted to Time to have the beginning of the day time.
• The created_before parameter will be casted to Time to have the end of the day time.

Why:

Issue.where('created_at <= ?', '2019-01-01') 

This query will not match the issues created at 2019-01-01 10:00

After this change:

Issue.where('created_at <= ?', '2019-01-01 23:59:59') 

I had to update a few feature test cases because they became "unstable".

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