Payments add modal form

* adds help texts to add payment form
* adds removing button for payments
* refactors user fetching into BaseForm
* adds generic RemoveModalForm which is intended to be used for every modal form which shall remove something
* adds translations
* removes unused html
* prepares payment amount field to be able to process german inputs like '1.000,50' which is not the international default

templates/modal/modal_form.html

@@ -1,4 +1,4 @@
-{% load i18n %}
+{% load i18n l10n %}
 {% comment %}
     A generic modal form template which is based on django-bootstrap-modal-forms package
     https://pypi.org/project/django-bootstrap-modal-forms/
@@ -15,18 +15,9 @@
     </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 %}">
-            <label for="{{ field.id_for_label }}">{{ field.label }}</label>
-            {{ field }}
-            {% for error in field.errors %}
-            <p class="help-block">{{ error }}</p>
-            {% endfor %}
-        </div>
-        {% endfor %}
-        -->
+        <article>
+            {{ form.form_caption }}
+        </article>
         {% include 'table/generic_table_form_body.html' %}
     </div>
     <div class="modal-footer">