MIS template updates - Date-field widget modifications


**Functionalities implemented:**

1) Updated: ```management/commands/create_schema.py```
 
   - New column added in ATs.csv, i.e. ```validators```
     - It is of type list
     - index-0: data-type required for template-tag filters on templates
     - index-1: class-name that helps in identifying which type of date-widget; 
       - Currently of two types: 
         1) only year & 
         2) other changable year & month with day 
     - index-2: placeholder value
     
   - So, code added for parsing ```validators``` field of AttributeType
     
2) Updated: ```templates/ndf/html_field_widget.html```
 
   - validator field's value(s) for each datetime attribute is assigned to it's corresponding input type element's respective attribute(s), that is:
     - validator.0: value attribute (in date template-tag filter)
     - validator.1: class attribute
     - validator.2: placeholder attribute

3) Updated: ```templates/ndf/person_create_edit.html```
 
   - Javascript code added for two type of classes:
     1. date_month_day_year - for field such as dob (date_of_birth)
     2. date_year - for fields such as 12_passing_year (students), degree_passing_year (voluntary teachers)