Resolve "Follow-up from "Implement Duo Enterprise Trial Widget"

What does this MR do and why?

The purpose of this MR is to handle some of the items that came up in the initial Duo Enterprise design MR per the follow up issue.

This MR

• adds space between the widget and the navbar
• updates the trial widget spec to test the component output rather than the library (spec will be built out more in subsequent issues)
• adds trial threshold to Trial Widget constant

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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

Before	After

How to set up and validate locally

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

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK.

2. Sign in. Create new group.

3. In rails console create / update new GitLab subscription for your new group:

   bin/rails c

   namespace = Group.last
   gitlab_subscription = namespace.gitlab_subscription # find or create a new one if needed
   gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

4. To test Duo Enterprise Widget

   bin/rails c

   add_on_purchase = GitlabSubscriptions::AddOnPurchase.create(subscription_add_on_id: GitlabSubscriptions::AddOn.find_by(name: 'duo_enterprise').id, namespace: Group.last, started_at: 55.days.ago, expires_on: (Date.today + 5.days), trial: true, quantity: 1, purchase_xid: '1', organization_id: 1)

5. Run trial widget spec

Related to #498710 (closed)