Review Board has optional support for a licensed product, Power Pack,

which provides a number of additional features including Document

Review. Power Pack currently provides its own management UI and

expiration warning banners for its license and trial states.
By having this live exclusively in Power Pack, it's difficult to always

seamlessly represent state in our UI, particularly as our UI changes,

since Power Pack supports a wide range of Review Board versions. It also

requires any other extensions to provide all their own UI, making it

challenging if we wanted to introduce licenses for anything else.
This change introduces the first steps in centralizing some of this.

Extensions are still responsible for their own licensing mechanisms, but

now they can provide a representation of their licensed state via new

BaseLicenseProvider and LicenseInfo classes.
BaseLicenseProvider can be subclassed and implemented by extensions to

provide lookup for license information, state for license checks, and

actions that users can perform.
LicenseInfo represents common information on an active, expired, or

trial license, for the purposes of display and management in the UI.
None of this is responsible for license enforcement. It's purely an

informational source, and will be used to better help licensed customers

see their license states in the Admin UI and to keep track of

expiring/expired licenses or trial states.
Upcoming changes will introduce UI for viewing and managing licenses, as

needed by licensed extensions.
For now, license management is gated by an Experimental feature flag,

but this will be enabled fully in 7.1.

Tested this with the beginnings of the UI work and the License Provider

implementation for Power Pack 6. There isn't much actual logic in this

change to test or execute at this time.