Matrix: Setup bot, add PIN verification

PIN is verified but not used currently. Works a little different than
the others, you input your matrix user ID and then the PIN is sent to
you. The bot doesn't support E2EE, so the bot being the first one to
message ensures the chat is unencrypted.

lang/form/en-us.json

@@ -19,7 +19,8 @@
         "confirmationRequiredMessage": "Please check your email inbox to verify your address.",
         "yourAccountIsValidUntil": "Your account will be valid until {date}.",
         "sendPIN": "Send the PIN below to the bot, then come back here to link your account.",
-        "sendPINDiscord": "Type {command} in {server_channel} on Discord, then send the PIN below via DM to the bot."
+        "sendPINDiscord": "Type {command} in {server_channel} on Discord, then send the PIN below via DM to the bot.",
+        "matrixEnterUser": "Enter your User ID, press submit, and a PIN will be sent to you. Enter it here to continue."
     },
     "notifications": {
         "errorUserExists": "User already exists.",
@@ -27,6 +28,7 @@
         "errorTelegramVerification": "Telegram verification required.",
         "errorDiscordVerification": "Discord verification required.",
         "errorInvalidPIN": "PIN is invalid.",
+        "errorUnknown": "Unknown error.",
         "verified": "Account verified."
     },
     "validationStrings": {