Add refresh tokens for persistent login, logout button

the main JWT is stored temporarily, whereas the refresh token is stored
as a cookie and can only be used to obtain a new main token. Logout
button adds token to blocklist internally and deletes JWT and refresh
token from browser storage.

data/templates/admin.html

@@ -258,6 +258,9 @@
                 <button type="button" class="btn btn-primary" id="openSettings">
                     Settings <i class="fa fa-cog"></i>
                 </button>
+                <button type="button" class="btn btn-danger" id="logoutButton" style="display: none;">
+                    Logout <i class="fa fa-sign-out"></i>
+                </button>
             </div>
             <div class="card mb-3 linkGroup">
                 <div class="card-header">Current Invites</div>