Custom away prompt

Created by: artemruts

Description

Fixes https://github.com/sourcegraph/sourcegraph/issues/19832

Todo

• confirm functionality with @quinnkeast
• confirm general implementation with FE Plats
• mark as ready to review

Demo

https://user-images.githubusercontent.com/1319181/114132189-e2777b00-98d1-11eb-82e1-83562316d403.mov