add moderation_action type

src/babillard.ml

@@ -1,6 +1,22 @@
 open Db
 include Bindings
 
+type moderation_action =
+  | Ignore
+  | Delete
+  | Banish
+
+let moderation_action_to_string = function
+  | Ignore -> "ignore"
+  | Delete -> "delete"
+  | Banish -> "banish"
+
+let moderation_action_from_string = function
+  | "ignore" -> Some Ignore
+  | "delete" -> Some Delete
+  | "banish" -> Some Banish
+  | _ -> None
+
 type thread_data =
   { subject : string
   ; lng : float