Made first_name and last_name mandatory in UserAdd

[khavinshankar]

Fixes #2362

Made first name and last name fields mandatory and also added validations to first name and last name fields in user creation form.

first name and last name fields can only contain letters from a-z, A-Z and space.

Note: first name and last name fields are only made mandatory in the frontend and it is still optional in the backend

[nihal467]

@khavinshankar @gigincg test failed, it is not accepting name

[nihal467]

test approved