Add invitation template filter (#11368)

Because Django. This is just a wrapper for working around Django templates and calling ``Invitation.can_revoke_invitation`` with arguments.

readthedocs/invitations/templatetags/invitations.py

@@ -0,0 +1,14 @@
+"""Invitation template filters."""
+
+from django import template
+
+from readthedocs.invitations.models import Invitation
+
+register = template.Library()
+
+
+@register.filter
+def can_revoke_invitation(user, object):
+    if isinstance(object, Invitation):
+        return object.can_revoke_invitation(user)
+    return False