Fix recently-introduced logical issues around invitations

* metasrht/templates/register.html: Without the fix, even users with
an invitation would be told "Registration is currently closed."

* metasrht/templates/tabs.html: The invites tab should be visible when
registration is closed, not when open.

metasrht/templates/register.html

@@ -17,7 +17,7 @@
 <p>Registration is disabled because sr.ht authentication is managed by a
   different service. Please contact the system administrator for further
   information.</p>
-{% elif allow_registration() %}
+{% elif allow_registration() or invite_hash %}
 <div class="row">
   <div class="col-md-10 offset-md-1">
     {% include "blurb.html" %}

metasrht/templates/tabs.html

@@ -28,7 +28,7 @@
   {% endif %}
 </li>
 {% endif %}
-{% if allow_registration() %}
+{% if not allow_registration() %}
 <li class="nav-item invite-tab">
   {{ link("/invites", "invites ({})".format(current_user.invites)) }}
 </li>