Lower strict_min_version to 140.0 so the extension can be used with Firefox ESR.

Fixed an error in the extension due to a missing background script.

Fix an error where the list of vouches websites would be cleared when an error occurred while crawling the web of trust.

There are no UI changes, only the discoverability protocol changed. Instead of using /.well-known/human.json, the file is now discovered via a <link rel="human-json" src="..."> element, according to the new version of the protocol (0.1.1). This allows the protocol to be used in sites with multiple users, for example:

https://example.com/~alice
https://example.com/~bob

Since each user can have their own human.json file.