Fixes: https://github.com/go-gitea/gitea/pull/6357

To support High-DPI displays, generate images at 4 times their displayed
resolution, which should be enough to support all current devices
according to https://material.io/devices/.

* Add makefile task to generate images from SVG and change to new logo

* use absolute path on generate-images

Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com>