Make AvatarRenderedSizeFactor configurable and set it to 3 (#17951)

Save a bit of bandwidth by only requesting 3-times the rendered avatar
size. Factor 4 is only really beneficial on a handful of mobile phones
and I don't think they are the primary device we design for.

Configurability contributed by zeripath.

Fixes: https://github.com/go-gitea/gitea/pull/17422
Fixes: https://github.com/go-gitea/gitea/issues/16287

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -710,6 +710,7 @@ Define allowed algorithms and their minimum key length (use -1 to disable a type
 - `AVATAR_MAX_WIDTH`: **4096**: Maximum avatar image width in pixels.
 - `AVATAR_MAX_HEIGHT`: **3072**: Maximum avatar image height in pixels.
 - `AVATAR_MAX_FILE_SIZE`: **1048576** (1Mb): Maximum avatar image file size in bytes.
+- `AVATAR_RENDERED_SIZE_FACTOR`: **3**: The multiplication factor for rendered avatar images. Larger values result in finer rendering on HiDPI devices.
 
 - `REPOSITORY_AVATAR_STORAGE_TYPE`: **default**: Storage type defined in `[storage.xxx]`. Default is `default` which will read `[storage]` if no section `[storage]` will be a type `local`.
 - `REPOSITORY_AVATAR_UPLOAD_PATH`: **data/repo-avatars**: Path to store repository avatar image files.