Support SSL for web interface

Thanks to @MasterofJOKers, here is basic support for SSL:
You need to create your key and certificate and copy them to the `data`
directory, named `ssl.cert` and `ssl.key`.
Then, set `PAPERLESS_USE_SSL` in `docker-compose.env` to `true`.

Solves #576.

docker-compose.env.example

@@ -16,6 +16,7 @@
 # different from PAPERLESS_OCR_LANGUAGE (default=eng), which defines the
 # default language used when guessing the language from the OCR output.
 # PAPERLESS_OCR_LANGUAGES=deu ita
+# PAPERLESS_USE_SSL=false
 
 # You can change the default user and group id to a custom one
 # USERMAP_UID=1000