Make the footer stick to the bottom of the page

And simplified it a little, making it a single footer instead
of upper-footer and lower-footer.
Use default buttons to confirm/cancel forms

Changed all forms I could find to use the same style of buttons
to confirm and cancel the action. Can still be adjusted, but
at least now all forms use the same style.
Yield more than just :headers in layouts, put assets in the right place

Now there's a specific place for javascripts, stylesheets and general headers.
Previously when using the layout "no_sidebar" a css was being included at
the bottom of the document, causing the page to be shown initially with a
space for the sidebar and making the other components to be placed incorrectly.