Set new Diffusion repository URIs to No I/O by default

We do not canonically host repositories in Diffusion anymore, thus all new Diffusion repos created are observed (and in addition sometimes mirrored). So far setting up an observed repo required 18 clicks to change the I/O mode of all 6 default URIs before being able to successfully create the external URI to observe.

Test Plan: Create a new Diffusion repository, then go to its URIs. See that the default URI's I/O is "No I/O" instead of "Read/Write".

Bug: T224753