Adds a footer block to base.

I've added a foot block to base.html

This is needed if you easily want to be able to modify all parts of your template.

I extend base.html to create my own custom layout, and without this change it is not possible to easily add a footer (or really design a custom template without completely rewriting base.html myself).

No tests should be required, though I am a bit unsure about the {% -%} syntax and what it does, I figured the footer should allow the same as the content block so I used the same syntax.