Pin all requirements in requirements.txt

The list of top-level dependencies is moved to requirements-app.txt,
which is used by `make freeze-requirements` to generate the full
list of requirements in requirements.txt.

(See alphagov/notifications-api#1938 for details.)

requirements-app.txt

@@ -0,0 +1,24 @@
+# Run `make freeze-requirements` to update requirements.txt
+# with package version changes made in requirements-app.txt
+
+ago==0.0.92
+Flask==1.0.2
+Flask-WTF==0.14.2
+Flask-Login==0.4.1
+
+blinker==1.4
+pyexcel==0.5.8
+pyexcel-io==0.5.7
+pyexcel-xls==0.5.7
+pyexcel-xlsx==0.5.6
+pyexcel-ods3==0.5.2
+pytz==2018.5
+gunicorn==19.8.1
+whitenoise==3.3.1  #manages static assets
+eventlet==0.22.1
+notifications-python-client==4.10.0
+
+# PaaS
+awscli-cwlogs>=1.4,<1.5
+
+git+https://github.com/alphagov/notifications-utils.git@29.3.2#egg=notifications-utils==29.3.2