License Providers that register custom actions can now handle them in

requests from the client. This is done by registering the action name in

the License Provider's custom_actions set and then adding a

handle_<actionid>_action() method to handle the response.
BaseLicenseProvider.call_action() has been added to invoke these

actions. This does the work formerly done by the view, making it more

generally-useful outside of HTTP handling. This takes the action ID,

action request data, the license information to operate off of, and an

optional HTTP request (which may be needed by some actions).
The action data may be any arbitrary JSON-safe data. Previously, when

the view handled an action, it expected keys and values in the HTTP POST

data to be used as arguments, but now it requires an explicit

action_data JSON field, which can then be deserialized and passed to

action handlers.
The built-in actions (license-update-check, process-license-update,

and upload-license) now live on BaseLicenseProvider instead of the

view, giving call_action() access to them.

Unit tests pass.
Manually tested the license update flow and some custom actions.