[FIX] web: always use browser timezone

With the new views, we stopped using the browser timezone to
display the dates in Odoo, and we used the timezone defined on
the User profile instead. When loading the webclient, the
timezone offset was put into the session and used to display all
dates. This wasn't a good idea.

The given offset was computed for the current time, meaning that
it may be incorrect for specific dates (e.g. with the daylight
saving, the UTC offset of today is not the same as 6 months ago).
Moreover (but less likely), as the offset was stored in the
session, it wasn't recalculated afterwards. So if the offset
actually changed during the session (e.g. from or to daylight
saving time), the displayed dates were incorrect until the user
reloaded the page.

With this rev., we don't retrieve the offset from the server
anymore and we use the browser timezone again (like before the new
views). However, we keep the computation of the offset (on the fly)
in the session, so that it can be mocked in the test environment.

addons/web/static/src/js/fields/basic_fields.js

@@ -450,7 +450,7 @@ var FieldDateTime = FieldDate.extend({
      */
     _getValue: function () {
         var value = this.datewidget.getValue();
-        return value && value.add(-this.getSession().tzOffset, 'minutes');
+        return value && value.add(-this.getSession().getTZOffset(value), 'minutes');
     },
 
     /**
@@ -460,7 +460,7 @@ var FieldDateTime = FieldDate.extend({
      * @private
      */
     _makeDatePicker: function () {
-        var value = this.value && this.value.clone().add(this.getSession().tzOffset, 'minutes');
+        var value = this.value && this.value.clone().add(this.getSession().getTZOffset(this.value), 'minutes');
         return new datepicker.DateTimeWidget(this, {defaultDate: value});
     },
 
@@ -471,7 +471,7 @@ var FieldDateTime = FieldDate.extend({
      * @private
      */
     _renderEdit: function () {
-        var value = this.value && this.value.clone().add(this.getSession().tzOffset, 'minutes');
+        var value = this.value && this.value.clone().add(this.getSession().getTZOffset(this.value), 'minutes');
         this.datewidget.setValue(value);
         this.$input = this.datewidget.$input;
     },