Improve UI in security reports

What does this MR do?

• Make inline buttons right aligned/floated
• Introduce actual colored icons for the - and + and have less screaming red text

Screenshots (if relevant)

How to test locally

@dimitrieh I don't have these reports set up locally, in order to test this I hacked around a little: in this file ee/app/assets/javascripts/vue_merge_request_widget/mr_widget_options.js, do the following

1. line 44 change the content to return true:

shouldRenderDockerReport() {
  return true;
},

1. line 338 change the content with:

this.mr.setDockerReport({
      unapproved: [
        'CVE-2017-12944',
        'CVE-2017-16232',
      ],
      vulnerabilities: [
        {
          vulnerability: 'CVE-2017-12944',
          namespace: 'debian:8',
          severity: 'Medium',
        },
        {
          vulnerability: 'CVE-2017-16232',
          namespace: 'debian:8',
          severity: 'Negligible',
        },
        {
          vulnerability: 'CVE-2014-8130',
          namespace: 'debian:8',
          severity: 'Negligible',
        },
      ],
    });

1. Just to check the success icon, on line 408 add :resolved-issues="mr.dockerReport.approved"

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

#4310 (closed)