[ImgBot] Optimize images (#361)

*Total -- 20,587.83kb -> 17,156.76kb (16.67%)

/assets/lightning.gif -- 4,208.49kb -> 2,771.07kb (34.16%)
/assets/flare.gif -- 6,525.52kb -> 5,502.98kb (15.67%)
/assets/readme-banner.png -- 112.85kb -> 98.39kb (12.81%)
/assets/border.gif -- 6,004.89kb -> 5,373.86kb (10.51%)
/assets/combatlog.gif -- 2,949.14kb -> 2,658.24kb (9.86%)
/assets/crystals.gif -- 786.95kb -> 752.21kb (4.41%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>
Co-authored-by: ImgBotApp <ImgBotHelp@gmail.com>

Author: imgbot[bot]