feat: implement 'is_default' org field (#12142)

The first organization created is now marked as "default". This is
to allow "single org" behavior as we move to a multi org codebase.

It is intentional that the user cannot change the default org at this
stage. Only 1 default org can exist, and it is always the first org.

Closes: https://github.com/coder/coder/issues/11961

coderd/apidoc/docs.go

@@ -10342,6 +10342,7 @@ const docTemplate = `{
             "required": [
                 "created_at",
                 "id",
+                "is_default",
                 "name",
                 "updated_at"
             ],
@@ -10354,6 +10355,9 @@ const docTemplate = `{
                     "type": "string",
                     "format": "uuid"
                 },
+                "is_default": {
+                    "type": "boolean"
+                },
                 "name": {
                     "type": "string"
                 },