Security Dashboard API hookup

What does this MR do?

This MR hooks up the dashboard to the actual API and makes a few UX tweaks along the way.

The page is still hidden behind a redirect for the following reasons:

1. The pagination on the API is broken #7970 (closed)
2. There's no documentation to link out to yet #7954 (closed)
3. We're waiting on the next version of gitlab SVG to be merged into CE, then EE gitlab-ce!22299
4. The icon for new issue is not yet merged gitlab-svgs#141 (closed)

Screenshots

Seeding / local dev instructions

1. Pull down branch 6709-group-security-dashboard-api-integration-fe-ee and run the project with the GDK.

2. Edit this line and change 10.times do to 50.times do (or any number greater than 20)

3. Remove or comment out this line

4. Go into the rails console bundle exec rails console

5. Seed the dummy vulnerabilities load 'db/fixtures/development/20_vulnerabilities.rb'

6. Go to the local instance of gitlab and create a group if you don't already have one

7. Go to http://localhost:3001/groups/[your group name]/-/security/dashboard

What are the relevant issue numbers?

#6709 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?