dont use has_permissions to check for platform admins

platform_admin is a separate concept to permissions, so by removing the
checks for it from the current_user.has_permissions function, we can
simplify things greatly. We already record on the user whether they're
a platform admin anyway.

app/templates/admin_template.html

@@ -44,7 +44,7 @@
           {% if current_user.is_authenticated %}
             <li><a href="{{ url_for('main.documentation') }}">Documentation</a></li>
             <li><a href="{{ url_for('main.user_profile') }}">{{ current_user.name }}</a></li>
-            {% if current_user.has_permissions(admin_override=True) %}
+            {% if current_user.platform_admin %}
               <li><a href="{{ url_for('main.platform_admin') }}">Platform admin</a></li>
             {% endif %}
             <li><a href="{{ url_for('main.sign_out')}}">Sign out</a></li>

app/templates/org_nav.html

@@ -1,8 +1,6 @@
 <nav class="navigation">
   <ul>
   	<li><a href="{{ url_for('.organisation_dashboard', org_id=current_org.id) }}">Dashboard</a></li>
-    {% if current_user.has_permissions(admin_override=True) %}
-      <li><a href="{{ url_for('.manage_org_users', org_id=current_org.id) }}">Team members</a></li>
-    {% endif %}
+    <li><a href="{{ url_for('.manage_org_users', org_id=current_org.id) }}">Team members</a></li>
   </ul>
 </nav>

app/templates/views/edit-email-template.html

@@ -19,7 +19,7 @@
           {{ textbox(form.name, width='1-1', hint='Your recipients won’t see this', rows=10) }}
           {{ textbox(form.subject, width='1-1', highlight_tags=True, rows=2) }}
           {{ textbox(form.template_content, highlight_tags=True, width='1-1', rows=8) }}
-          {% if current_user.has_permissions(admin_override=True) %}
+          {% if current_user.platform_admin %}
              {{ radios(form.process_type) }}
           {% endif %}
           {{ page_footer(

app/templates/views/edit-sms-template.html

@@ -20,7 +20,7 @@
         </div>
         <div class="column-two-thirds">
           {{ textbox(form.template_content, highlight_tags=True, width='1-1', rows=5) }}
-          {% if current_user.has_permissions(admin_override=True) %}
+          {% if current_user.platform_admin %}
             {{ radios(form.process_type) }}
           {% endif %}
           {{ page_footer(

app/templates/views/service-settings.html

@@ -250,7 +250,7 @@
 
     {% endif %}
 
-    {% if current_user.has_permissions(admin_override=True) %}
+    {% if current_user.platform_admin %}
 
       <h2 class="heading-medium">Platform admin settings</h2>
       {% call mapping_table(