Add short-eared owl as the Owltide mascot and icon

I've settled on a short-eared owl (Asio Flammeus) to serve as the mascot
and icon for Owltide. This is not a settled and final decision, but
rather a branding that's good enough for now.

To create the mascot I sketched some owls that I have saved to
/assets/mascot/owl-sketch.png and then selected one I liked.  Going
clockwise from the top right the sketches were based on the reference
photos [1], [2], and [3] found by searching for "owl" on Wikimedia
Commons.  I then lightly refined it to work as a small icon.

To avoid confusing terms that only make sense for software the artworks
are licensed under the Creative Commons Attribution-ShareAlike 4.0
License.  This has a similar spirit to the AGPL and ensures that should
improvements be made upon them they can be incorporated in the project.

[1]: https://commons.wikimedia.org/wiki/File:Athene_cunicularia_-near_Goiania,_Goias,_Brazil-8_edit.jpg
[2]: https://commons.wikimedia.org/wiki/File:Hibou_des_marais.jpg
[3]: https://commons.wikimedia.org/wiki/File:Uhu-muc.jpg

assets/mascot/owl-icon-128.png.license

@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: © 2025 Hornwitser <code@hornwitser.no>
+SPDX-License-Identifier: CC-BY-SA-4.0

assets/mascot/owl-icon-32.png.license

@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: © 2025 Hornwitser <code@hornwitser.no>
+SPDX-License-Identifier: CC-BY-SA-4.0

assets/mascot/owl-icon-64.png.license

@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: © 2025 Hornwitser <code@hornwitser.no>
+SPDX-License-Identifier: CC-BY-SA-4.0

assets/mascot/owl-icon.png.license

@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: © 2025 Hornwitser <code@hornwitser.no>
+SPDX-License-Identifier: CC-BY-SA-4.0

assets/mascot/owl-sketch.png.license

@@ -0,0 +1,2 @@
+SPDX-FileCopyrightText: © 2025 Hornwitser <code@hornwitser.no>
+SPDX-License-Identifier: CC-BY-SA-4.0