Improve production schedule

[NicoPennec]

Problem

• When loading Production Schedule loading, two backup calls are executed without changing data.
• In the modal to edit a milestone, a too long name will crash in silent, without error message.
• UX glitches on add/edit milestone buttons positioned at the start of the month.

Solution

• Avoid useless save call on init Production Schedule.
• Add a max length to the milestone name field, and handle error in modal on failure.
• Rework css of milestone.