Merge sponsors.py into supporters app, frontpage.py into views.py

Just a small structural change so that the related functionality is grouped together.

Merge "supporter" and "supporters" apps

It's a little confusing having both a "supporter" and "supporters" app. This is
a fairly minor change to move the sustainer banner urls/views into the existing
"supporters" app. The functionality isn't directly related, but it is all about
sustainers.

podjango: Add "Podcast" model to support multiple podcasts

Each Cast (episode) can belong to one or more Podcast, allowing episodes to be
shared between podcasts. This enables us introductory episodes to be delivered
in their own feed, but also included in the main "The Corresponding Source"
feed.

This required adding an additional `podcast_slug` argument to most views. The
date archive views were dropped because they're not linked to from anywhere.

Added a `podcasts` view as an index of all available Podcasts.

contacts: Remove ContactEntry and add Unsubscription

This change removes the unused `ContactEntry` model and the `subscribe` view and
replaces it with an `Unsubscription` model and an `unsubscribe` view. It works
similarly, but is intended to be used with the `list-unsubscribe` and
`list-unsubscribe-post` headers.

Move the content templates/files into `conservancy/content`

These were previously intermingled with the static content in `conservancy/static`.

Remove unnecessary `admin.autodiscover()`

Since Django 1.7 this is done automatically.

Rewrite the `index` view to avoid risk of path traversal

I've simplified this view by removing the custom HTTP error handlers, Python 3.5
exception handling and adding documentation.

Move Python code out of the "conservancy/static" directory

Having Python code in "conservancy/static" is a bit suprising to people familiar
with Django. The name "static" is usually reserved for assets like CSS, JS and
images.

I'm moving `conservancy/static/views.py` to `conservancy/views.py` and removing
`conservancy/static/__init__.py`.

Remove unused ccs-upload feature

Use the Source uploads now go to a NextCloud anonymous upload.

Move /cast/ to /cast/the-corresponding-source/

This makes room for addition of other podcasts later.

Switch CCS upload, copyright assignment, FOSSY and podcast components to use path()

This is the newer approach to URL routing in Django.