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

docs/api/organizations.md

@@ -123,6 +123,7 @@ curl -X POST http://coder-server:8080/api/v2/organizations \
 {
   "created_at": "2019-08-24T14:15:22Z",
   "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
+  "is_default": true,
   "name": "string",
   "updated_at": "2019-08-24T14:15:22Z"
 }
@@ -163,6 +164,7 @@ curl -X GET http://coder-server:8080/api/v2/organizations/{organization} \
 {
   "created_at": "2019-08-24T14:15:22Z",
   "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",
+  "is_default": true,
   "name": "string",
   "updated_at": "2019-08-24T14:15:22Z"
 }