Base URL configuration options (#4749)

* Improve construct_absolute_url method

- Look for hard-coded site URL if provided
- Otherwise look for specified site URL
- Otherwise look at the provided request object

* Refactor existing code which used base URL setting

* Update docs

* Validate that a provided base URL is valid

InvenTree/part/templatetags/inventree_extras.py

@@ -222,8 +222,8 @@ def inventree_splash(**kwargs):
 
 @register.simple_tag()
 def inventree_base_url(*args, **kwargs):
-    """Return the INVENTREE_BASE_URL setting."""
-    return InvenTreeSetting.get_setting('INVENTREE_BASE_URL')
+    """Return the base URL of the InvenTree server"""
+    return InvenTree.helpers.get_base_url()
 
 
 @register.simple_tag()