Reposition mr widget correctly

What does this MR do and why?

This MR does two things:

1. Repositions the widget container and puts it inside the correct container.
2. Hides the widget container behind a feature flag (realised this one was missing - it didn't create any problem so far because the widget container itself also checks for the feature flag, but still it's better to hide the root component directly).

Screenshots or screen recordings

before	after

How to set up and validate locally

1. You'll need an EE License
2. You'll need to have runners enabled (See $2408961 for setting up a runner)
3. Enable :refactor_security_extension to turn this feature on.

echo "Feature.enable(:refactor_security_extension)" | rails c

1. Import https://gitlab.com/gitlab-examples/security/security-reports
2. Create a new MR by editing a file
3. Go to the MR page and check the widget
4. It should be positioned as shown in the screenshot

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383233 (closed)