Payments add modal form

* adds modal form for adding payments
* generalizes generic_table_form.html for table-form-like usage in modal_form.html
* adds css enhancements for focused input fields
* adds BaseModalForm as specification to BaseForm, which inherits the BSModalForm class as well
* adds translations

templates/modal/modal_form.html

@@ -15,6 +15,7 @@
     </div>
 
     <div class="modal-body">
+        <!--
         <article class="mb-5">{{form.form_caption}}</article>
         {% for field in form %}
         <div class="form-group{% if field.errors %} invalid{% endif %}">
@@ -25,6 +26,8 @@
             {% endfor %}
         </div>
         {% endfor %}
+        -->
+        {% include 'table/generic_table_form_body.html' %}
     </div>
     <div class="modal-footer">
         <button type="submit" class="btn btn-default">{% trans 'Continue' %}</button>