Favicon for Links 2.1.0

1. Added file health check.
2. Improved js code efficiency.
3. Added a user specific setting to disable Favicon for Links. This will allow users that don't like the favicon next to links to disable it.
   The setting is placed in the account preferences page.

This is a major update, which breaks backward compatibility.
Any customizations to the addon won't work and will need to be re-applied.
The templates or/and style properties of the addon that you may have edit, should be reverted.