[ADD] web_calendar_slot_duration: customizable calendar slots timespan

This module extends the functionality of backend calendars to support custom
slot durations and to allow you to provide more specific UX regarding event
duration and snapping.

@Tecnativa TT28201

[UPD] Update web_calendar_slot_duration.pot

[UPD] README.rst

web_calendar_slot_duration/static/src/js/calendar_model.js

@@ -0,0 +1,22 @@
+/* Copyright 2021 Tecnativa - Jairo Llopis
+ * License LGPL-3.0 or later (https://www.gnu.org/licenses/lgpl). */
+
+odoo.define("web_calendar_slot_duration.CalendarModel", function (require) {
+    "use strict";
+
+    var CalendarModel = require("web.CalendarModel");
+
+    CalendarModel.include({
+        /**
+         * @override
+         */
+        _getFullCalendarOptions: function () {
+            var result = this._super.call(this, arguments);
+            result.slotDuration =
+                this.data.context.calendar_slot_duration ||
+                result.slotDuration ||
+                "00:30:00";
+            return result;
+        },
+    });
+});