issue-diff: rework issue date check to use date in .changes to properly allow 30 days

[jberry-suse]

As I had done in my original approach, the .changes file needs to be parsed in order to use the date from the change entry header to skip issues included in the last 30 days to allow for sync with Factory. The current code uses the OBS API (as suggested), /source/$project/$package?view=issues which only includes the dates relevant to the bug itself.

I'll take a look at the original parsing code and either a) find a python equivalent that already exists, b) port the code to python, or c) utilize via external script.