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/table/generic_table_form.html

@@ -0,0 +1,36 @@
+{% load i18n  %}
+
+{% block head %}
+    {{ form.media }}
+{% endblock %}
+
+{% block body %}
+<div class="container">
+    <h4>
+        {{ form.form_title }}
+    </h4>
+    {% if form.form_caption is not None %}
+    <small>
+        {{ form.form_caption }}
+    </small>
+    {% endif %}
+    <form method="post" action="{{ form.action_url }}">
+        {% csrf_token %}
+        {% include 'table/generic_table_form_body.html' %}
+        <div class="row">
+            <div class="col-md">
+                <a href="{{ form.cancel_redirect }}">
+                    <button class="btn btn-default" type="button" title="{% trans 'Cancel' %}">{% trans 'Cancel' %}</button>
+                </a>
+            </div>
+            <div class="col-md d-flex justify-content-end">
+                <button class="btn btn-default" type="submit" title="{% trans 'Save' %}">{% trans 'Save' %}</button>
+            </div>
+        </div>
+    </form>
+</div>
+{% endblock %}
+
+{% block footer %}
+    {{ form.media }}
+{% endblock %}