Add timely reminder button*

* Add a check in Timely command that removes the reminder button if there already is a timely reminder in DB.
* Add ReminderType in db/models. As well as migrations.
* Set Normal reminders to be of `ReminderType.User`, and Timely reminders to be of `ReminderType.Timely`

src/Nadeko.Bot.Db/Models/Reminder.cs

@@ -9,4 +9,11 @@ public class Reminder : DbEntity
     public ulong UserId { get; set; }
     public string Message { get; set; }
     public bool IsPrivate { get; set; }
+    public ReminderType Type { get; set; }
+}
+
+public enum ReminderType
+{
+    User,
+    Timely
 }