Added a footer sidebar and removed max-width limitation (dev)

[bernattorras]

Changes proposed in this Pull Request

• Added the "Footer - Above credits" sidebar
• Removed the max-width restriction from the footer

Testing instructions

• Go to Appearance > Widgets and confirm that there's a sidebar named "Footer - Above credits"
• Add content there
• Open any page from the Frontend and confirm that this content gets rendered in the footer, above the colophon credits
• Confirm that the content of the footer is taking the 100% of its width.

Mentions #21

[bernattorras]

Merging into develop to check the changes from the staging site.