Fix csv export file name

What does this MR do?

Currently when you hit the export button it will download a file called download. This MR provides a meaningful name for that file. The format is as follows:

csv-export-{{isDateTime}}.csv

For instance;

csv-export-2020-04-13T13_33_56+0300.csv.

Steps to test

1. Go to Project Security Dashboard (you'll need an EE license and the first_class_vulnerabilities feature flag enabled).
2. On top right above the filters there is an export. Click on that.
3. Expect the file name to be like above.

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 #197494 (closed)