Add all the boilerplate for a standalone app.
Change deprecated argument name.
Create a cache subdirectory per month.

Directories with tens of thousands of files are impracticaly. This also
provides a convenient way to clear less relevant portions of the cache
(eg. old photos).
Enforce preview size in CSS.

This allows using double resolution for retina displays.
Fix translations. Commit .mo file.
PEP8 cleanup.