Adds Blinker dependency which is missing after recent changes (#15182)

This PR fixes a problem introduced by #14144

This is a very weird and unforeseen issue. The change introduced a
new import from flask `before_render_template` and this caused
flask to require `blinker` dependency, even if it was not
specified before as 'required' by flask. We have not seen it
before, because changes to this part of the code do not trigger
K8S tests, however subsequent PRs started to fail because
the setup.py did not have `blinker` as dependency.

However in CI image `blinker` was installed because it is
needed by sentry. So the problem was only detectable in the
production image.

This is an ultimate proof that our test harness is really good in
catchig this kind of errors.

The root cause for it is described in
https://stackoverflow.com/questions/38491075/flask-testing-signals-not-supported-error

Flask support for signals is optional and it does not blinker as
dependency, but importing some parts of flask triggers the need
for signals.

(cherry picked from commit 437850b)

Author: potiuk

setup.cfg

@@ -82,6 +82,7 @@ install_requires =
     alembic>=1.2, <2.0
     argcomplete~=1.10
     attrs>=20.0, <21.0
+    blinker
     cached_property~=1.5
     # cattrs >= 1.1.0 dropped support for Python 3.6
     cattrs>=1.0, <1.1.0;python_version<="3.6"