[ImgBot] Optimize images

*Total -- 1,790.60kb -> 1,397.06kb (21.98%)

/redis-session.png -- 473.34kb -> 365.00kb (22.89%)
/tests.png -- 361.50kb -> 279.29kb (22.74%)
/message-db.png -- 653.05kb -> 509.86kb (21.93%)
/users-db.png -- 273.46kb -> 217.15kb (20.59%)
/db-diagram.png -- 29.26kb -> 25.76kb (11.97%)

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

Author: ImgBotApp