added AGPL license text to header of template file

mediagoblin_libreplanet/templates/libreplanet/join.html

@@ -1,3 +1,20 @@
+{#
+# MediaGoblin for LibrePlanet
+# Copyright (C) 2017 Andrew Engelbrecht <andrew@fsf.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#}
 
 <a href="http://fsf.org/jfb">
     <img id="join-button" src="//static.fsf.org/nosvn/join.en.png" alt="Join now" />