Fixes checks that reported the wrong result. Google Drive links were reported as "sign-in required" even when they opened fine without an account, deleted files were not recognised outside English, and deleted Dropbox links reported "couldn't verify" instead of "broken". Checks are now decided on the provider's HTTP response rather than page wording, so the result no longer depends on the language the provider replies in.

Adds a "Grant provider access" button. Firefox does not grant add-on site access at install, so first-run checks previously failed with a generic network error and no way to fix it.