Fix spacing between columns

What does this MR do?

Fixes the spacing between columns in the vulnerability list in the security dashboards. The new Security Dashboard uses the gl-table component from gitlab-ui. That manages the spacings between the columns much better. However, since this dashboard will be disabled as soon as we release the new security dashboard, I didn't refactor it to use gl-table component.

The fix was to change the columns to a percentage width. That should fix the spacing in the Group and Instance Level dashboards. However, it also changes slightly the Project Level dashboard because we're reusing the same component in all three dashboards. I don't think it's a big deal if we slightly increase the spacing in the Project Level but I will leave this to the UX people. You can see the screenshots below.

NOTE In order to view this dashboard, you'll need to disable the feature flag if it's enabled. You can do so by:

# In the gitlab folder execute:
bin/rails console
# Once the console is ready execute:
Feature.disable(:first_class_vulnerabilities)
# After waiting a few seconds refresh the page
# To reenable the feature flag:
Feature.enable(:first_class_vulnerabilities)
# Type `exit` to exit
exit

Screenshots

How it looks on the Project Level after the changes:

This is the Group Level:

This is the Instance Level:

Mobile view:

This is how the mobile view was before:

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