[IMP] web: add RemainingDays field widget

This widget can be used on date and datetime fields. In readonly,
it displays the delta (in days) between the value of the field and
today. In edit, it behaves like a regular date(time) widget.

Part of task 2195254

doc/reference/javascript_reference.rst

@@ -1686,7 +1686,7 @@ order.
         <field name="datetimefield" options='{"datepicker": {"daysOfWeekDisabled": [0, 6]}}'/>
 
 - daterange (FieldDateRange)
-    This widget allow user to select start and end date into single picker.
+    This widget allows the user to select start and end date into a single picker.
 
     - Supported field types: *date*, *datetime*
 
@@ -1702,6 +1702,13 @@ order.
 
         <field name="start_date" widget="daterange" options='{"related_end_date": "end_date"}'/>
 
+- remaining_days (RemainingDays)
+    This widget can be used on date and datetime fields. In readonly, it displays
+    the delta (in days) between the value of the field and today. It edit, it
+    behaves like a regular date(time) widget.
+
+    - Supported field types: *date*, *datetime*
+
 - monetary (FieldMonetary)
     This is the default field type for fields of type 'monetary'. It is used to
     display a currency.  If there is a currency fields given in option, it will