[Admin & Feedback] Optional talk start/end date time

[HugoGresse]

Fix #659

• talk dates are now optional
• Feedback: if all talks are without dates, hide the date selection on the feedback app & display them from the root (example.com/mytalk) without the day at the end. This get one step closer to Make urls prettier using a slug instead of ids #535
• Feedback: if one talk or more has no dates but others has, display a "Without date" in the menu, selected by default
• Admin: date time picker is now a dialog with a Clear/Cancel/OK button inside
• Admin: date time field no longer show autocomplete help

Eg event with some talks without dates

eg event with no date on talks

[cypress]

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Test summary

7 • 0 • 0 • 0

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Run details

Project	open-feedback
Status	Passed
Commit	7cf3a20
Started	May 21, 2020 12:22 PM
Ended	May 21, 2020 12:23 PM
Duration	01:13 💡
OS	Linux Ubuntu Linux - 16.04
Browser	Electron 80

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