lib/analysis: Extracted from_seconds_of_day() function
i18n/de: Fixed typos
lib/string: Fixed re.sub() usage

The parameters were switched, which led to wrong results...
lib/string: Fixed non_alpha_re regexp
tests: Added unittest for skylines.lib.string
SQL: Reset empty logger IDs

The broken lib.string caused empty logger IDs, which have to be set to
NULL instead so that the igc_headers URL can be called to reanalyze them.