Add dark mode

[felipecrs]

Dark mode was removed with the introduction of the new Web UI.

This brings it back, with a few improvements.

• Added a toggle button to switch themes between Auto, Light, and Dark modes.
• The Auto mode respects the system theme preference. It's the default mode.
• The Network Graph page required some minor adjustments to look nice in Dark mode.
• The Config Editor page also received a few tweaks:
  • The new default themes are CloudEditor and CloudEditor Dark. I believe they look better than the previous defaults. Let me know if you disagree. You can try all themes here.
  • Removed the 80 characters ruler line, as it clutters the UI and it is not really relevant.
  • Change so that when the editor is opened, it will no longer select all the text by default.

This PR also:

• Upgraded vis-network from v9.1.9 to v10.0.2.
• Upgraded ace from v1.33.1 to v1.43.4.

dark.mode.mp4