Rework file organization and prepare for i18n support

Inspired by CritiqueBrainz code organization, introduce the use of Flask blueprints


The footer looks kinda odd now (the padding within the gray box is uneven). Possibly we should also steal the CritiqueBrainz styling here. :) https://critiquebrainz.org/

The Picard site doesn't have consistent margins, but it'd probably look fine for the footer to have the same width as the "features" div, with the footer links and language selector floated the left/right respectively.