Precompute action permissions for table pages

- Extract reusable helpers for database and table action permission preloading.
- Precompute those permissions before building table-page HTML data.
- Document the default table actions plugin.

docs/plugins.rst

@@ -280,6 +280,15 @@ If you run ``datasette plugins --all`` it will include default plugins that ship
                 "query_actions"
             ]
         },
+        {
+            "name": "datasette.default_table_actions",
+            "static": false,
+            "templates": false,
+            "version": null,
+            "hooks": [
+                "table_actions"
+            ]
+        },
         {
             "name": "datasette.events",
             "static": false,