Support localized app metadata

Merged username-removed-26331 requested to merge grote/repomaker:19-localized-metadata into master

This adds experimental support for localized app metadata within four logically separated commits.

For now, it only support Summary and Description. Also these keys are about to be changed and then need to be adapted once that change made it to repositories.

Due to limitations of the underlying translation library, untranslated apps do not show up in Django Admin.

Closes #19 (closed)