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 these request are read-only or read-write is currently tricky. Since these repositories can't really cluster yet, this shouldn't matter too much today.

Test Plan:

• Without any config changes, viewed repositories via web UI and pushed/pulled via SSH and HTTP.
• Made all nodes in the cluster read-only by disabling "writable", pulled and hit the web UI (worked), tried to push via SSH and HTTP (got errors about read-only).
• Put everything back, pulled and pushed.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13120, T10883

Differential Revision: https://secure.phabricator.com/D19357