[FIX] board,web: edit custom views in place

Before this commit, whenever we do an operation on any of these custom views,
such as folding it, the custom view was saved by creating a new one.

This behaviour comes from a lost feature to undo operations on custom views.
Since we do not have this feature anymore, it makes no sense to create new
custom views on save, instead of applying the changes in place.

With this commit, custom views are edited in place.

Fixes #23712

addons/board/static/src/js/dashboard.js

@@ -169,8 +169,8 @@ var DashBoard = form_common.FormWidget.extend({
             board.columns.push(actions);
         });
         var arch = QWeb.render('DashBoard.xml', board);
-        this.rpc('/web/view/add_custom', {
-            view_id: this.view.fields_view.view_id,
+        this.rpc('/web/view/edit_custom', { // do not forward-port > saas-15
+            custom_id: this.view.fields_view.custom_view_id, // do not forward-port > saas-15
             arch: arch
         });
     },

addons/web/controllers/main.py

@@ -930,14 +930,18 @@ class DataSet(http.Controller):
 
 class View(http.Controller):
 
-    @http.route('/web/view/add_custom', type='json', auth="user")
-    def add_custom(self, view_id, arch):
-        CustomView = request.session.model('ir.ui.view.custom')
-        CustomView.create({
-            'user_id': request.session.uid,
-            'ref_id': view_id,
-            'arch': arch
-        }, request.context)
+    @http.route('/web/view/edit_custom', type='json', auth="user")
+    def edit_custom(self, custom_id, arch):
+        """
+        Edit a custom view
+
+        :param int custom_id: the id of the edited custom view
+        :param str arch: the edited arch of the custom view
+        :returns: dict with acknowledged operation (result set to True)
+        """
+
+        custom_view = request.env['ir.ui.view.custom'].browse(custom_id)
+        custom_view.write({ 'arch': arch })
         return {'result': True}
 
 class TreeView(View):