Make footer elements customizable

[Samuel Mehrbrodt]

This mainly applies for the Songs footer.

The footer can become very large if you have multiple authors, songbook info, CCLI license etc.
Also you might want to display some parts of the footer larger than others (title vs copyright) -> HTML should be allowed

So you should be able to configure the footer. I suggest we add a text box with placeholders for all footer elements (like {{title}}, {{copyright}}, {{songbooks}}, {{ccli_license}} etc).

I suggest to use Pystache (Python implementation of the Mustache template engine): https://github.com/defunkt/pystache
This would also allow loops (for lists like authors, songbooks) and conditional display.