Limit assignee drop-down selector scope

On the task details page "Add a sub-task" pop-up modal contains
drop-down (the SELECT HTML element) with "form-user_id" as the `name`
and the `id` attribute value. This element is in conflict with another,
hidden INPUT element with the same `name` and the `id` value. This
causes an issue where clicking the "Me" (Assignee) link / shortcut (A)
element doesn't select your name in the Assignee SELECT element.

Avoiding conflict by changing the SELECT element `id` value is not
possible as the SELECT element is generated using `FormHelper::select()`
function which uses the same value for attribute `name` and `id`.

Given the assignee shortcut feature is used only in two places: edit
task and edit sub-task dialog, the simplest solution is to select the
closest SELECT element to the link / shortcut (A) element or limit the
scope to the currently opened dialog.

This fix uses the latter approach by limiting the scope of targeted
selector.

assets/js/src/Task.js

@@ -11,7 +11,7 @@ Kanboard.Task.prototype.onPopoverOpened = function() {
     // Assign to me
     $(document).on("click", ".assign-me", function(e) {
         var currentId = $(this).data("current-id");
-        var dropdownId = "#" + $(this).data("target-id");
+        var dropdownId = "#modal-box #" + $(this).data("target-id");
 
         e.preventDefault();