Add license to the top of each source file

frontend/src/App.css

@@ -1,3 +1,21 @@
+/*
+Source code for the Substrate Telemetry Server.
+Copyright (C) 2021 Parity Technologies (UK) Ltd.
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU 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 General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <https://www.gnu.org/licenses/>.
+*/
+
 .App {
   text-align: left;
   font-family: Roboto, Helvetica, Arial, sans-serif;