Update policy editor layout css

What does this MR do and why?

Describe in detail what your merge request does and why.

Update policy editor layout css

• make preview take up the whole screen on less than xlarge
• update button css

Changelog: changed

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Scenario	Before	After	After - smaller screen
New policy
Existing project policy			Same as small screen above
Existing group policy			-

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Upload a GitLab Ultimate license
2. Navigate to http://gdk.test:3443/groups/flightjs/-/security/policies/new?type=scan_execution_policy and shrink the screen
3. Navigate to http://gdk.test:3443/groups/flightjs/-/security/policies/new?type=scan_execution_policy and shrink the screen
4. Create a policy, merge the MR, navigate to http://gdk.test:3443/groups/flightjs/-/security/policies, edit the policy, and shrink the screen

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 #383048 (closed)