Added an option to change the emoji on the locked tabs.

- Change the default icons to simpler ones.
- Split the lock URLs to 3 lists (foreground, background, excluded) in the option settings.
- Automatically adds the manually locked URLs into the option settings.

- Changed "Auto-Lock" setting to partial match from exact match.
- Adds lock emoji to document title when locking.
- Merged icon settings (foreground/background to "enabled")
- Fixed bugs on saving preferences.

Minor fixes to support Firefox for Android.

Migrated to WebExtensions.