Add option to forcibly show the trial status popover

What does this MR do?

Adds a prop option for forcing the trial status popover to start initially shown.

When that prop is present, the popover title will also have an × (close) button for dismissing the popover. When the popover is forcibly shown, it can only be dismissed by clicking on that × button (or by navigating to a different page).

This MR also makes sure that the onShown event & related tracking still happens when the popover is programmatically shown in this way.

Screenshots

Before (not forcibly shown)	After (forcibly shown)
Screen_Recording_2021-06-22_at_3.12.25_PM	Screen_Recording_2021-06-24_at_4.06.04_PM

How to test locally

1. Enable billing plans & trials:

   ApplicationSetting.first.update(check_namespace_plan: true)

2. Enable the feature flag for this experiment:

   Feature.enable_percentage_of_time(:show_trial_status_in_sidebar, 100)

3. Grab a group you like to use locally, get its id, and then give that group a trial:

   group = Group.find(<group_id>)
   plan = Plan.create_with(title: 'Ultimate').find_or_create_by(name: 'ultimate')
   days_remaining_in_trial = 14
   subscription = group.gitlab_subscription || GitlabSubscription.new(namespace: group)
   subscription.hosted_plan = plan
   subscription.trial_starts_on = Date.current.advance(days: days_remaining_in_trial - 30)
   subscription.trial_ends_on = Date.current.advance(days: days_remaining_in_trial)
   subscription.trial = true
   subscription.save
   
   # then you should see...
   group.reload.trial_active? # => true

4. Then navigate to that group in the UI.

5. You can also play around with how many days are left in the group’s trial:

   group = Group.find(<group_id>)
   days_remaining_in_trial = 3
   group.gitlab_subscription.update(
     trial_starts_on: Date.current.advance(days: days_remaining_in_trial - 30),
     trial_ends_on: Date.current.advance(days: days_remaining_in_trial)
   )

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #333604 (closed) & #334936 (closed)