[WIP] Fix planning modal issues

[f2cmb]

Fixes partially #13943

• issue with datepicker behaviour was fixed by adding the class suggested by @orthagh
• JS error was fixed by changing scope issue that cause the message (const was reinjected on every opening of modal) : use now of var and "sub-JS-functionnalities" are now wrapped to re-use form var

TODO : fixe dropdown non-stuck to their label zone, the behaviour is erratic, i can reproduce on one window opened, and not on a second on same browser, setup, widht and heigth... Also cannot reproduce point 3 of what is described on the original issue. Will update if able to reproduce those.

[trasher]

I wonder if this should be fixed on v11 only; fix will maybe differs.