Trial and license purchases inside GitLab EE

What does this MR do?

• Create trial license key and immediately add it to the current server
• Allow creation of trial license only if the GitLab instance doesn't have a license
• Update the license detail page for the following states:
  • When there is no license
  • When there is an active trial license
• Buttons related to EE features should be disabled but not hidden
• Write specs

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots

Admin User Welcome (without License)

Desktop View

Mobile View

Standard User Welcome (without License)

Desktop View

Mobile View

Admin User Trial Banner

Trial Expiring/Expired

Desktop View

Mobile View

Trial License Modal

Default View - Collapsed

Expanded View

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2500 (closed) #2501 (closed)