Append rather than override classes in e-mail and export forms

We recently had a number of issues to append rather than override classes on form elements. By appending classes we allow modules and themes to preprocess additional classes onto the elements. This patch cleans up the last remaining places in the 7.x-3.x branch. The 7.x-4.x branch has already had these changes made as part of #2033885: Redirection location inline textfield should not be wrapped in normal form wrappers.

Related issues:
#2031055: Theme cannot add classes via preprocess to webform_advanced_total_submit_limit_form
#2031053: Theme cannot add classes via preprocess to webform_advanced_submit_limit_form
#2031057: Theme cannot add classes via preprocess to webform_advanced_redirection_form
#2030869: Theme cannot add classes via preprocess webform date
#2030885: Append rather than override classes in theme_preprocess_webform_time()