[luci-notify] Use project scoped account for access via gitiles.
[luci-notify] Use project scoped account for access via gitiles.

luci-notify service will use project scoped accounts to access
repositories via gitiles since per-service service accounts will
not maintain their access after project migration.

Bug: 967696
Change-Id: Iddba01a85874f27c073cfc113315989333dc6860
R: a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1635880
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Felix Matenaar <6f9f11a7d9d9f9d70470ce23ff837316447e1df6@chromium.org>

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