Private message refactoring: fix up various template issues. Added a template tag to generate the URL to send a PM to a user. This replaced the {% url 'messages-compose_to' username %} in various templates.

.user_photo {
   margin: 1em;
   display: inline-block;
}
.owner_photo {
   border: 1px solid gray;
}
.owner_photo img {
   margin: auto;
   width: 100%;
}
hr.footer_divider {
   margin-top: 1.5em;
}