R1:6556536d0615

Allow repository cluster bindings to be marked as not "writable", making them read-only

Summary:
Depends on D19356. Fixes T10883. Ref T13120.

- Add a "writable" property to the bindings, defaulting to "true" with a nice dropdown.
- When selecting hosts, allow callers to request a writable host.
- If the caller wants a writable host, only return hosts if they're writable.
- In SVN and Mercurial, we sometimes return only writable hosts when we //could// return read-only hosts, but figuring out if…