Use placeholder to display default/new value in grey/black in the form.

Removed JS code that used to show default value of class fields in grey on value Instance edit form and used to change it to black text when user changed the value. Replaced that functionality by using :placeholder attribute of text_field_tag. Deleted any related CSS and JS methods.
Merge pull request #1100 from h-kataria/automate_instance_display_default_value_in_form_changes

Use placeholder to display default/new value in grey/black in the form.