Stop (unnecessarily) enforcing UTF8 for set_locale form

Doing so provides us with no advantages, and has the side effect of
triggering [this bug](rack/rack#673) in Rack
when said form is submitted in browsers that do not support UTF-8

Note that that enforcement is intended as a protection against
users submitting Latin-1 values when we expect unicode; since this form
supports no user-provided values and doesn't persist any data, that's
not an issue we need to worry about.

Note also that the only browsers for which this error is likely to be
generated are browsers that we do not officially support, so an argument
could be made for simply ignoring this error. But, I figure, I already
got the fix right here.

dashboard/app/helpers/locale_helper.rb

@@ -79,7 +79,7 @@ def serve_fonts?
   end
 
   def i18n_dropdown
-    form_tag(locale_url, method: :post, id: 'localeForm', style: 'margin-bottom: 0px;') do
+    form_tag(locale_url, method: :post, id: 'localeForm', style: 'margin-bottom: 0px;', enforce_utf8: false) do
       (hidden_field_tag :return_to, request.url) + (select_tag :locale, options_for_select(options_for_locale_select, locale), onchange: 'this.form.submit();')
     end
   end

dashboard/app/views/layouts/_footer.html.haml

@@ -23,7 +23,7 @@
 
       .span3{style: 'text-align: right;' + (full_width ? 'width: 140px; margin-left: 0px' : '')}
 
-        = form_tag(locale_url, method: :post, id: 'localeForm', style: 'margin-bottom: 0px;') do
+        = form_tag(locale_url, method: :post, id: 'localeForm', style: 'margin-bottom: 0px;', enforce_utf8: false) do
           = hidden_field_tag :return_to, request.url
           = select_tag :locale, options_for_select(options_for_locale_select, locale), onchange: 'this.form.submit();'
         %small.dim