Update COPYRIGHT and rename/update LICENSE.txt.

Try to fix GitHub’s license detection algorithm.

COPYRIGHT

@@ -1,7 +1,7 @@
 django-newsletter: Django app for managing multiple mass-mailing lists with
 both plaintext as well as HTML templates (and TinyMCE editor for HTML messages),
 images and a smart queueing system all right from the admin interface.
-Copyright (C) 2008-2013 Mathijs de Bruin
+Copyright (C) 2008-2020 Mathijs de Bruin and other contributors
 
 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU Affero General Public License as

LICENSE → LICENSE.txt

@@ -1,7 +1,7 @@
                     GNU AFFERO GENERAL PUBLIC LICENSE
                        Version 3, 19 November 2007
 
- Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
  Everyone is permitted to copy and distribute verbatim copies
  of this license document, but changing it is not allowed.
 
@@ -633,8 +633,8 @@ the "copyright" line and a pointer to where the full notice is found.
     Copyright (C) <year>  <name of author>
 
     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
+    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,
@@ -643,7 +643,7 @@ the "copyright" line and a pointer to where the full notice is found.
     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/>.
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 Also add information on how to contact you by electronic and paper mail.
 
@@ -658,4 +658,4 @@ specific requirements.
   You should also get your employer (if you work as a programmer) or school,
 if any, to sign a "copyright disclaimer" for the program, if necessary.
 For more information on this, and how to apply and follow the GNU AGPL, see
-<http://www.gnu.org/licenses/>.
+<https://www.gnu.org/licenses/>.