Replace jquery-datetimepicker with native date input (#11684)

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.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/less/_repository.less

@@ -1309,10 +1309,6 @@
         textarea {
             height: 200px;
         }
-
-        #deadline {
-            width: 150px;
-        }
     }
 
     &.compare.pull {