Nativefier v50.0.0

[BREAKING] Block some external URLs from opening outside of Nativefier (fix #1459)
This is a security fix, all users are encouraged to upgrade.

This will, for security, block loading 2 kinds of external (out of Nativefier) URLs:
One: using dubious URL schemes, two: including nasty characters.
Blocking URLs will be accompanied by a window explaining what's going on,
and linking to a discussion thread where you can report false positives.
Hopefully not BREAKING much (the behavior should now be aligned with
what browsers do), but web weirdness happens. Shout and we'll tweak.

Also,

• Fix double-navigation to pages (fix #1452)
• Upgrade cli+app dependencies
• Bump default Electron to 19.0.17 (from .14), with security fixes
  • https://github.com/electron/electron/releases/tag/v19.0.15
  • https://github.com/electron/electron/releases/tag/v19.0.16
  • https://github.com/electron/electron/releases/tag/v19.0.17