refactor requirements and make htmlmin optional

src/requirements/dev.txt

@@ -0,0 +1,2 @@
+flake8
+isort

src/requirements/htmlmin.txt

@@ -0,0 +1 @@
+django-htmlmin>=0.9,<0.10

src/requirements/mysql.txt

@@ -0,0 +1,2 @@
+requirements.txt
+mysqlclient

src/requirements/postgres.txt

@@ -0,0 +1,2 @@
+requirements.txt
+psycopg2

src/requirements/production-extra.txt

@@ -0,0 +1 @@
+django-htmlmin>=0.9,<0.10

src/requirements/production.txt

@@ -0,0 +1,6 @@
+Django>=1.9,<1.10
+django-bootstrap3>=6.2,<6.3
+django-compressor==2.0
+csscompressor
+djangorestframework>=3.4,<3.5
+django-filter>=0.14,<0.15