Invites: unique email/ID requirement

"Require unique ..." Settings (`require_unique` in relevant sections of
config.ini) are now available for email/discord/telegram/matrix. An
error is shown on the invite form if a non-unique address/ID is used.
This was on my kanban without a link to an issue, so i'm guessing it was
requested on Discord.

lang/form/en-us.json

@@ -24,6 +24,8 @@
     "notifications": {
         "errorUserExists": "User already exists.",
         "errorInvalidCode": "Invalid invite code.",
+        "errorAccountLinked": "Account already in use.",
+        "errorEmailLinked": "Email already in use.",
         "errorTelegramVerification": "Telegram verification required.",
         "errorDiscordVerification": "Discord verification required.",
         "errorMatrixVerification": "Matrix verification required.",