This adds a new dedicated Licenses page in the Admin UI for seeing all

current registered licenses across License Providers. These indicate the

state of the licenses, expiration information, provide license

management, actions, and custom line items.
Each license is represented by a License model and LicenseView view,

backed by state provided by the License Provider. Custom models, views,

and data can be provided as needed.
Some basic infrastructure is set up for custom actions and license

checks. To ease review, those capabilities will be introduced in a

follow-up change.

Python and JavaScript unit tests passed.
Tested this with some custom License Providers registered in extensions

as tests, and with Power Pack ported over to License Providers. Verified

that all the state and status updates introduced in this change function

as expected.