Reference dev.yml's 'orphan' environment variables from dev.env

This is to elaborate on the proposal made in #1082. 

Contain 'orphan' variables scattered all around dev.yml in a separate dev.env file which can be stored in VCS likewise the development-specific variables it defines are.

N.B. `django` service's `USE_DOCKER=yes` variable left intact for its use to be isolated within the service only.

{{cookiecutter.project_slug}}/dev.env

@@ -0,0 +1,4 @@
+POSTGRES_USER={{cookiecutter.project_slug}}
+POSTGRES_PASSWORD={{cookiecutter.project_slug}}
+POSTGRES_DB={{cookiecutter.project_slug}}
+

{{cookiecutter.project_slug}}/dev.yml

@@ -10,8 +10,8 @@ services:
     volumes:
       - postgres_data_dev:/var/lib/postgresql/data
       - postgres_backup_dev:/backups
-    environment:
-      - POSTGRES_USER={{cookiecutter.project_slug}}
+    env_file:
+      - dev.env
 
   django:
     build:
@@ -20,8 +20,9 @@ services:
     command: /start-dev.sh
     depends_on:
       - postgres
+    env_file:
+      - dev.env
     environment:
-      - POSTGRES_USER={{cookiecutter.project_slug}}
       - USE_DOCKER=yes
     volumes:
       - .:/app
@@ -40,8 +41,8 @@ services:
       dockerfile: ./compose/django/Dockerfile-dev
     depends_on:
       - postgres
-    environment:
-      - POSTGRES_USER={{cookiecutter.project_slug}}
+    env_file:
+      - dev.env
     volumes:
       - .:/app
     links: