Allow users to save a bit of text to be displayed at the end of every post on their blog.
Authors commonly include a signature, call to action, etc. at the end of their posts. Right now they can do that by copy-and-pasting or using templates in an outside tool. Building it into the platform would make it easy to add and importantly, easy to update.
* Provide the interface for inserting a new `collectionattribute` named `post_signature` (see Design section)
* Check for this value when loading an individual post (we should only show the signature on blog post pages, not the blog index)
* If it exists, append it (after a newline) to the normal post content before Markdown parsing, and parse it all
Add a new section in the blog's Customize page (maybe near / with the CSS and JS fields) that allows a user to enter plain text.
This should be a Write.as Extend feature, probably available for a $5 one-time fee.