Small improvements to chemdispenser UI
requested to merge pull/3969/ikeepmakingopenissuesandthenfixingtheminthesamedaybecauseirealizeihavenothingbettertodotofillmycrushinglethargy into Bleeding-Edge
Created by: 9600bauds
Fixes #3961 (closed), the "custom" field now remembers the last value entered. Also adds 15 and 25 to the possible amounts to choose from and fixes the 50 button being wonky. Rogue clamp