"Copy tasks from date" should be disabled for blocked dates

[jaragunde]

If you enable the block report feature, all the means to create new tasks are disabled for blocked dates, except the Copy tasks from selected date button.

The button will work if pressed, copying those tasks and adding them to the blocked day. Fortunately, the save button is disabled too so it has no effect but, still, it should be disabled.

[jaragunde]

Reviewed at PR #414:

commit 3dfa13c96c6386e366298f3117d769111351cd4d
Author: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Date:   Wed Jul 4 19:19:38 2018 +0200

    [#259] Disable copy tasks from date feature in blocked dates.
    
    `startDay` attribute was removed just because it's unnecessary.