safety-save: PEP-8 compliance
safety-save: Use UTC datetimes and ISO 8601 date formats

Let’s not invent yet another date format string. Use UTC dates to avoid
confusion if the user changes timezone.
safety-save: Use XDG user data directory for saving the files

Let’s not proliferate more dot-dirs.
plugin: Port to python3 loader

To work with recent versions of gedit.
safety-save: Do not disable the plugin when autosave is disabled

I always want my new documents auto-saved, but do not necessarily want
all documents auto-saved. There seems to be no harm in unconditionally
enabling auto-saving of new documents.

If the user really doesn’t want it enabled, they can disable the plugin.
Merge pull request #1 from pwithnall/fixes

Miscellaneous fixes