discord: fix admin-check for /inv

it was being checked in the EmailAddress record, only set if Jellyfin
login is disabled, or "access jfa-go" is checked for a
non-Jellyfin-admin user in Accounts. Instead, i've factored out the
actual auth code into a "canAccessAdminPage"-ish function, which is
called for this too. Should fix #378.

lang/telegram/en-us.json

@@ -13,6 +13,7 @@
         "languageSet": "Language set to {language}.",
         "discordDMs": "Please check your DMs for a response.",
         "sentInvite": "Sent invite.",
-        "sentInviteFailure": "Failed to send invite, check logs."
+        "sentInviteFailure": "Failed to send invite, check logs.",
+        "noPermission": "You do not have permissions for this action."
     }
 }