feat(generic): introduce generic menu entry

This commit introduces a template tag in the generic module that lists
all content types that inherit from the Generic Model but are not
Collections, Entities, Relations or History models.
This templatetag is then used to create a menu entry to manage those
types.

Closes: #1582

apis_core/generic/templates/base.html

@@ -0,0 +1,24 @@
+{% extends "base.html" %}
+{% load generic %}
+
+{% block main-menu %}
+  {{ block.super }}
+  <li class="nav-item dropdown">
+    <a href="#"
+       class="nav-link dropdown-toggle"
+       data-bs-toggle="dropdown"
+       role="button"
+       aria-haspopup="true"
+       aria-expanded="false">
+      Other
+      <span class="caret" />
+    </a>
+    <div class="dropdown-menu" aria-labelledby="navbarDropdown">
+      {% pure_genericmodel_content_types as content_types %}
+      {% for content_type in content_types %}
+        <a class="dropdown-item"
+           href="{{ content_type.model_class.get_listview_url }}">{{ content_type.name }}</a>
+      {% endfor %}
+    </div>
+  </li>
+{% endblock main-menu %}

apis_core/generic/templatetags/generic.py

@@ -1,4 +1,5 @@
 from django import template
+from django.apps import apps
 from django.contrib.contenttypes.models import ContentType
 
 from apis_core.core.templatetags.core import get_model_fields
@@ -61,6 +62,34 @@ def genericmodel_content_types():
     return genericmodels
 
 
+@register.simple_tag
+def pure_genericmodel_content_types():
+    """
+    Retrieve all models which inherit from GenericModel class
+    but are not Collections, Entities, Relations or History models
+    """
+    genericmodels = set(genericmodel_content_types())
+    parents = []
+    if apps.is_installed("apis_core.collections"):
+        collections = apps.get_app_config("collections")
+        parents.append(collections.models_module.SkosCollection)
+        parents.append(collections.models_module.SkosCollectionContentObject)
+    if apps.is_installed("apis_core.relations"):
+        relations = apps.get_app_config("relations")
+        parents.append(relations.models_module.Relation)
+    if apps.is_installed("apis_core.history"):
+        history = apps.get_app_config("history")
+        parents.append(history.models_module.APISHistoryTableBase)
+    if apps.is_installed("apis_core.apis_entities"):
+        entities = apps.get_app_config("apis_entities")
+        parents.append(entities.models_module.AbstractEntity)
+    genericmodels = filter(
+        lambda content_type: not issubclass(content_type.model_class(), tuple(parents)),
+        genericmodels,
+    )
+    return genericmodels
+
+
 @register.filter
 def get_attribute(obj, attribute):
     return getattr(obj, attribute, None)