Adds framework for "server health" display

- Adds global context object "system_healthy" (boolean)
- Framework for running system health checks
- Updated system info forms
- Displays warning next to user menu if system health errors exist

(cherry picked from commit ce6d626ab39147fc389de90815ca8baae2385d82)

InvenTree/templates/about.html

@@ -16,11 +16,6 @@
                 <div>
                     <table class='table table-striped table-condensed'>
                         <col width='25'>
-                        <tr>
-                            <td></td>
-                            <td>{% trans "Instance Name" %}</td>
-                            <td>{% inventree_instance_name %}</td>
-                        </tr>
                         <tr>
                             <td><span class='fas fa-hashtag'></span></td>
                             <td>{% trans "InvenTree Version" %}</td><td><a href="https://github.com/inventree/InvenTree/releases">{% inventree_version %}</a></td>