[feature request] start a new session with any session time from Digdag UI

[nemupm]

Digdag command can start a new session with any session time, like this.

digdag start myproj main --session "2016-01-01 00:00:00"

But, currently Digdag UI supports only to start a new session using current time as its session time. It would useful to start a new session with user input's session time, especially for unscheduled workflows.