[casviewer] add cas-viewer-dev back to app.yaml
[casviewer] add cas-viewer-dev back to app.yaml

This will allow us to deploy to dev via gae.py.
I sometimes need to deploy tainted version to confirm UI changes on dev.

Bug: 1114030
Change-Id: I0bb81d62694a50438354d6170e3a7ea6890b8213
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2440203
Auto-Submit: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go