TracAdmin: enable a version time to be unset from the WebAdmin or from the command-line.
With trac-admin, do: `trac-admin env version time name ""`
Similar fixes for milestone ''due'' and ''completed'' time in trac-admin (it was already working in WebAdmin).
Fixes #5529.
Authored by: cboos 2007-06-15
Parent: [r5693]
Child: [r5695]