Replace jquery-datetimepicker with native date input

[silverwind]

This removes the jQuery plugin as well as the associated config options. Native input[type=date] does not require a language attribute as it is localized by default, except for the placeholder attribute for which I currently piggy-back the repo.issues.due_date_form localization option.

Implementation should pretty much match GH. Of note is that Safari does not provide a UI for this input type, but I don't think providing one is neccessary and GH did not bother either.

Before:

After:

[CirnoT]

Something very weird is going on, when I select any date, the date set is always +1 day from what I selected.

[silverwind]

Works on my end:

Try it on GH's milestone create page, it should behave the same.

[CirnoT]

Works fine on GitHub; the issue isn't with native date picker, the date selected and sent to server is fine, but date saved in database is always +1 day.

[silverwind]

Weird. I see "2020-05-05" in the POST and on the rendered date afterwards.

[CirnoT]

Chromium sends
due_date: "2020-05-25T00:00:00.000Z"
And I see May 26 on rendered page.

[CirnoT]

In database it is set to 1590451199, that's why. Gitea timezone is +2 for me so 23:59 on 2020-05-25 is indeed 26th of May.

[silverwind]

Is this a Chromium bug? I thought type=date should always send yyyy-mm-dd.

[silverwind]

MDN says:

Value | A DOMString representing a date in YYYY-MM-DD format, or empty

[silverwind]

Chrome 83 does the right thing:

Why is there due_date in your example?

[CirnoT]

I'm testing it on issue due date, assuming it should behave the same way.

[silverwind]

Due date also sends with time info for me so there is a bug there that is not present on milestone.

[CirnoT]

Indeed, works fine on milestone. Wonder why due date sends time.

[silverwind]

It's wrongly passing the date to the JS Date contructor.

[silverwind]

I tried changing it to send yyyy-mm-dd format but the backend rejects that. Definitely a separate issue: #11686

[silverwind]

Any more reviews? This is ready from my side.

[CirnoT]

Need to resolve conflicts @silverwind

[silverwind]

Yeah, just noticed that, rebased.