Fix warnings in the Signet add-on for Firefox

Version 0.1.3 had no errors, no failed security tests, no failed extension tests. There were, however, 13 warnings. This ticket is to fix them.

Use "browser_specific_settings" instead of "applications".

Warning: The "applications" property in the manifest is deprecated and will no longer be accepted in Manifest Version 3 and above.
manifest.json
Manifest key not supported by the specified minimum Firefox for Android version

Warning: "strict_min_version" requires Firefox for Android 59, which was released before version 79 introduced support for "browser_action.default_icon".
manifest.json
browserAction.setIcon is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 30 column 3
browserAction.disable is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 31 column 3
browserAction.enable is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 33 column 3
browserAction.setIcon is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 36 column 4
browserAction.setIcon is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 48 column 7
browserAction.setIcon is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 50 column 7
browserAction.setIcon is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 53 column 6
browserAction.setIcon is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 56 column 5
browserAction.disable is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 160 column 3
browserAction.isEnabled is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 255 column 18
browserAction.openPopup is not supported in Firefox for Android version 59.0

Warning: This API is not implemented by the given minimum Firefox for Android version
background.js line 274 column 2

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