Update minimum Firefox verison to 56.0

The previous minimum version of 56.2 resulted in the build failing
validation when it was uploaded to the Firefox web store, because that
version doesn't exist. It was set to that version because a Firefox
fork uses it.

Instead the minimum version has been reduced so that we pass
validation. Unfortunately this will mean that a single incompatible
version of Firefox Mobile will allow the extension to be installed
(in theory), but there was no other way to avoid cutting off support
to WaterFox (the Firefox fork). The warning about this from the addons
linter can be ignored for now.

app/manifest.json

@@ -22,7 +22,7 @@
   "applications": {
     "gecko": {
       "id": "[email protected]",
-      "strict_min_version": "56.2"
+      "strict_min_version": "56.0"
     }
   },
   "default_locale": "en",