Bump werkzeug from 0.14.1 to 0.15.3 in /app

Bumps [werkzeug](https://github.com/pallets/werkzeug) from 0.14.1 to 0.15.3.
- [Release notes](https://github.com/pallets/werkzeug/releases)
- [Changelog](https://github.com/pallets/werkzeug/blob/master/CHANGES.rst)
- [Commits](pallets/werkzeug@0.14.1...0.15.3)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

app/requirements.txt

@@ -46,4 +46,4 @@ six==1.12.0
 SQLAlchemy==1.2.15
 urllib3==1.24.1
 waitress==1.1.0
-Werkzeug==0.14.1
+Werkzeug==0.15.3