Whitelist Footer Element

[cjeller1592]

Describe the bug

If you publish a post with the footer HTML element, it's stripped from the post.

Context: I am trying to use a web extension's embed feature (Quotebacks) that's code requires the manipulation of the embed's footer. Since the footer gets stripped from the post, the code cannot find the footer and doesn't render properly.

Steps to reproduce (if necessary)

Steps to reproduce the behavior:

1. Publish a post with a <footer></footer> element
2. Inspect the post in browser tools
3. Notice how the footer element in the post is gone

Expected behavior

The footer element should have stayed intact when you inspect the post.

[thebaer]

Thanks for reporting this! While we're at it, we might as well support the <header> element, too.

This should be a pretty straightforward fix -- simply add:

policy.AllowElements("header", "footer")

in this func:

https://github.com/writeas/writefreely/blob/5c94d23466dc3e7e1821b845d9d7db9294c48af4/postrender.go#L171-L184