Guest blog post support

[StephenChan]

Add a "guest author" User ForeignKey field to the blog post model, which gives edit access to a specific non-admin user for a specific post. This way, whenever someone writes a guest blog post for us, we don't have to convert from whatever format they're drafting in (e.g. Google doc) to CoralNet's blog post format. This can be particularly convenient when making various minor edits before publishing.

The editing interface is in the Django admin subsite, so we'd need to configure permissions to the admin subsite such that a non-admin user can log into it and only see the stuff they're supposed to see. Alternatively, we could move the editing interface out of the admin subsite.

I'd say this is low priority since guest blog posts are infrequent, and not every guest writer may be inclined to use the Markdown editor we use.