Fix updating of issues and milestones

This pull request fixes two issues with the scripts: 

The first is with quoting in the script for adding labels. The only part where quoting labels with spaces in the name is when updating an issue, so only quote the labels there. Quoting other places causes mismatches when detecting if a label already exists.

The second is with updating milestones. The REST API target for milestones is indexed by an arbitrary 'number' integer for the milestone, not by the milestone name. The script now determines this number and adds it to a dictionary for parsing through existing milestone names.

To test: run the scripts against a demo repository. Then change the parameters for the default labels/milestones, and run them again. Labels and milestones should update without error.