discord: add/move to slash commands

the version of the discord library with support for this isn't
necessarily stable, so normal ! commands will still be available. The
user is no longer DMed for the PIN, instead they type /pin <PIN>.

lang/telegram/en-us.json

@@ -4,10 +4,13 @@
     },
     "strings": {
         "startMessage": "Hi!\nEnter your Jellyfin PIN code here to verify your account.",
+        "discordStartMessage": "Hi!\n Enter your PIN with `/pin <PIN>` to verify your account.",
         "matrixStartMessage": "Hi\nEnter the below PIN in the Jellyfin sign-up page to verify your account.",
         "invalidPIN": "That PIN was invalid, try again.",
         "pinSuccess": "Success! You can now return to the sign-up page.",
         "languageMessage": "Note: See available languages with {command}, and set language with {command} <language code>.",
+        "languageMessageDiscord": "Note: set your language with /lang <language name>.",
+        "languageSet": "Language set to {language}.",
         "discordDMs": "Please check your DMs for a response."
     }
 }