register_permissions() plugin hook (#1940)

* Docs for permissions: in metadata, refs #1636
* Refactor default_permissions.py to help with implementation of #1636
* register_permissions() plugin hook, closes #1939 - also refs #1938
* Tests for register_permissions() hook, refs #1939
* Documentation for datasette.permissions, refs #1939
* permission_allowed() falls back on Permission.default, refs #1939
* Raise StartupError on duplicate permissions
* Allow dupe permisisons if exact matches

tests/conftest.py

@@ -90,6 +90,13 @@ def check_permission_actions_are_documented():
 
     def before(hook_name, hook_impls, kwargs):
         if hook_name == "permission_allowed":
+            datasette = kwargs["datasette"]
+            assert kwargs["action"] in datasette.permissions, (
+                "'{}' has not been registered with register_permissions()".format(
+                    kwargs["action"]
+                )
+                + " (or maybe a test forgot to do await ds.invoke_startup())"
+            )
             action = kwargs.get("action").replace("-", "_")
             assert (
                 action in documented_permission_actions