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.
