[ImgBot] Optimize images

*Total -- 65.49kb -> 52.08kb (20.48%)

/icons/today.png -- 11.64kb -> 5.51kb (52.63%)
/icons/users.png -- 8.10kb -> 6.40kb (20.94%)
/icons/settings.png -- 21.19kb -> 17.56kb (17.12%)
/icons/themes.png -- 24.57kb -> 22.60kb (8%)

Signed-off-by: ImgBotApp <[email protected]>

Author: ImgBotApp