[FIX] mail: improved message_fetch for moderated channels

There was two issues with the previous code:

	1. It was using a weak heuristic to determine whether the
	   current thread is a channel (ID is a number)
	2. For any channel, it was always passing `moderated_channel_ids`
	   with some values to the server, even though the channel
	   is not moderated at all.

This commit solves both issues by passing `moderated_channel_ids`
only for moderated channel. Also, this logic has been moved from
SearchableThread to Channel, since moderation makes only sense for
channels and channels are aware of whether they are moderated or
not.

addons/mail/static/src/js/models/threads/channel.js

@@ -357,6 +357,23 @@ var Channel = SearchableThread.extend(ThreadTypingMixin, {
             return lastSeenMessageID;
         });
     },
+    /**
+     * Override so that it tells whether the channel is moderated or not. This
+     * is useful in order to display pending moderation messages when the
+     * current user is either moderator of the channel or has posted some
+     * messages that are pending moderation.
+     *
+     * @override
+     * @private
+     * @returns {Object}
+     */
+    _getFetchMessagesKwargs: function () {
+        var kwargs = this._super.apply(this, arguments);
+        if (this.isModerated()) {
+            kwargs.moderated_channel_ids = [this.getID()];
+        }
+        return kwargs;
+    },
     /**
      * Get the domain to fetch all the messages in the current channel
      *

addons/mail/static/src/js/models/threads/searchable_thread.js

@@ -163,10 +163,7 @@ var SearchableThread = Thread.extend({
      */
     _fetchMessages: function (pDomain, loadMore) {
         var self = this;
-
         var domain = this._getThreadDomain();
-        var moderatedChannelIds = typeof(this._id) === 'number' ? [this._id] : false;
-
         var cache = this._getCache(pDomain);
         if (pDomain) {
             domain = domain.concat(pDomain || []);
@@ -179,11 +176,7 @@ var SearchableThread = Thread.extend({
             model: 'mail.message',
             method: 'message_fetch',
             args: [domain],
-            kwargs: {
-                moderated_channel_ids: moderatedChannelIds,
-                limit: this._FETCH_LIMIT,
-                context: session.user_context
-            },
+            kwargs: this._getFetchMessagesKwargs(),
         }).then(function (messages) {
             if (!cache.allHistoryLoaded) {
                 cache.allHistoryLoaded = messages.length < self._FETCH_LIMIT;
@@ -218,6 +211,19 @@ var SearchableThread = Thread.extend({
         }
         return this._cache[stringifiedDomain];
     },
+    /**
+     * Get the kwargs that are passed to the server with the 'message_fetch'
+     * RPC.
+     *
+     * @private
+     * @returns {Object}
+     */
+    _getFetchMessagesKwargs: function () {
+        return {
+            limit: this._FETCH_LIMIT,
+            context: session.user_context
+        };
+    },
     /**
      * Get the domain to fetch all the messages in the current thread
      *