Standardizing forms

form.html.erb

<%# currently on signup form %>
<%= f.label :first_name, :class => 't2 medium mb3 block grow' do %>
  First Name
  <%= f.text_field :first_name, :class => 'field-text mt1', :placeholder => 'No need for formalities' %>
<% end %>

<%# 
  One potential custom form helper. How many of the classes that are set above
  should be defaults?
%>
<%= f.wnw_text_with_label :first_name, :placeholder => 'No need for formalities' %>

<%# 
  Custom form helper that assumes fewer default classes. 
  Label includes "t2 medium block grow" already
  Input includes "field-text" already
%>
<%= f.wnw_text_with_label :first_name, :input_classes => 'mt1', 
    :label_classes => 'mb3', :placeholder => 'No need for formalities' %>