[catapult] use webapp2 from third party for dashboard
[catapult] use webapp2 from third party for dashboard

The webapp2 from google_appengine does not support py3. Using the one from third_party.

Bug: chromium:1221663
Change-Id: I02990b1c66e55aafcc9f3d8ff2a2f1fbbe586741
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/3044727
Reviewed-by: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>
Commit-Queue: Wenbin Zhang <45ad8a1ff616a38a2202fd35571df81897c5f14f@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult