Resolve "Adapt subscriptions page for free plans and trials"

What does this MR do?

This MR adapts the subscription table which was introduced in 11.6 for free plans and trials.

Some information doesn't apply to free plans and trials, so in those cases, the table has been adapted (by hiding specific rows/cells and showing certain information in other places) in order to avoid confusion.

Screenshots:

Subscription
Free
Trial

Howto create a "Gold trial" subscription locally:

Open up the rails console and execute the following (where my-trial-group is the group you want to create the Gold trial for):

plan = Plan.find_by_name('gold')
group = Group.find_by_path('my-trial-group')
group.create_gitlab_subscription!(hosted_plan: plan, seats: 100, start_date: Date.today, end_date: 1.year.from_now.to_date)
sub = group.gitlab_subscription
sub.update_attributes!(trial: true, trial_ends_on: 1.month.from_now)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8838

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
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• 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?
• Security reports checked/validated by reviewer

Closes #8838 (closed)