Allow saving bookmarks with authenticated URLs

Bookmarks behind authentication couldn't be saved.
The app didn't allow saving when URL metadata check failed, even
though Linkding accepts these URLs.
The error message still displays to inform users that metadata
couldn't be fetched, but it no longer blocks saving.

lib/i18n/strings_en.i18n.json

@@ -73,7 +73,7 @@
       "validateUrl": "Validate URL",
       "urlValid": "The URL is valid",
       "urlNotValid": "The URL is not valid",
-      "cannotCheckUrl": "The URL couldn't be checked",
+      "cannotCheckUrl": "The URL couldn't be checked, but you can save it anyway.",
       "invalidUrl": "Invalid URL",
       "url": "URL",
       "checkingUrl": "Checking URL...",