Set permissions with checkboxes, not yes/no inputs

The yes/no pattern didn’t work too well, because:
- it didn’t read naturally as a question and answer
- often users left them completely unclicked if they didn’t want to set
  the permission (rather than clicking no)

This commit changes both the invite and edit user pages to use
checkboxes to set permissions. If also rewords these pages to read more
naturally, and explain what the permissions mean.

This meant changing some of the view logic around invites and
persmissions, and I ended up refactoring a bunch of it because I found
it hard to understand what was going on.

app/templates/views/invite-user.html

@@ -1,5 +1,5 @@
 {% extends "withnav_template.html" %}
-{% from "components/yes-no.html" import yes_no %}
+{% from "components/checkbox.html" import checkbox %}
 {% from "components/textbox.html" import textbox %}
 {% from "components/page-footer.html" import page_footer %}
 
@@ -10,23 +10,15 @@ Manage users – GOV.UK Notify
 {% block maincolumn_content %}
 
   <h1 class="heading-large">
-    {{ user.name or user.email_localpart or "Invite a team member" }}
+    {{ "Invite a team member" }}
   </h1>
 
   <div class="grid-row">
     <form method="post" class="column-three-quarters">
 
-      {{ textbox(form.email_address, hint='You must use an email address from a central government organisation', width='1-1', safe_error_message=True) }}
+      {{ textbox(form.email_address, hint='Must be from a central government organisation', width='1-1', safe_error_message=True) }}
 
-      <fieldset class='yes-no-wrapper'>
-        <legend class='heading-small'>
-          Permissions
-        </legend>
-        <span class="form-hint">All team members can see message history</span>
-        {{ yes_no(form.send_messages, form.send_messages.data) }}
-        {{ yes_no(form.manage_service, form.manage_service.data) }}
-        {{ yes_no(form.manage_api_keys, form.manage_api_keys.data) }}
-      </fieldset>
+      {% include 'views/manage-users/permissions.html' %}
 
       {{ page_footer('Send invitation email') }}