Pad out date with '0' if needed in OC.filepicker. Fixes #8802

See: https://github.com/owncloud/core/issues/8802

To test, you can install the Music app, go to Personal settings, click the box to choose your music location. 

All date elements should be at least 2 digits long. Yeah should be 4 digits.

@jancborchardt @wakeup