Query for vulnerability history

What does this MR do?

This MR adds a method to Vulnerability that returns historical data.

Given a range of days up to the present day (for example, from 30 days ago to today), the method returns counts of detected and un-fixed (un-dismissed and un-resolved) vulnerabilities grouped by day and severity.

Example response where we have 3 severities (high, medium, and low) and we've asked for the past 2 days of data:

[
  { "day" => "2019-10-30", "severity" => "high", "count" => 1},
  { "day" => "2019-10-30", "severity" => "medium", "count" => 3},
  { "day" => "2019-10-30", "severity" => "low", "count" => 4},
  { "day" => "2019-10-31", "severity" => "high", "count" => 3},
  { "day" => "2019-10-31", "severity" => "medium", "count" => 1},
  { "day" => "2019-10-31", "severity" => "low", "count" => 4}
]

#213625 (closed)

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