COPYING: Add copyright notice for the whole repository

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COPYING

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+Unless otherwise stated this repository is copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>, and is distributed under The GNU Affero General Public License Version 3, you should have received a copy of the license file in AGPL-3.
+
+---
+
+The following files are copyright © 2019 shitposter.club, and are distributed under the Creative Commons Attribution-ShareAlike 4.0 International license, you should have received a copy of the license file into CC-BY-SA-4.0.
+
+priv/static/images/pleroma-fox-tan.png
+priv/static/images/pleroma-fox-tan-smol.png
+priv/static/images/pleroma-tan.png
+
+---
+
+The following files are copyright © 2017-2019 Pleroma Authors <https://pleroma.social/>, and are distributed under the Creative Commons Attribution-ShareAlike 4.0 International license, you should have received a copy of the license file into CC-BY-SA-4.0.
+
+priv/static/images/avi.png
+priv/static/images/banner.png
+priv/static/images/city.jpg
+priv/static/instance/thumbnail.jpeg