Closes #26514

This fixes the case when the user wants to add a label. The user has to use the dropdown’s filter input to look for a label and click it in order to see the bug.

Step to reproduce

- Select at least two issues, one label should be present in all issues, other label should be present in at least one.

- On the label dropdown: Deselect label that is present in all issues, look for another issue using the filter input and click it.

- Click on `Update issues`

Before: Unmarked label were kept on selected issues.
Now: Unmarked label is removed from selected issues

This ports some code over from EE to reduce conflicts

Opening the user dropdown currently sends 2 requests. This has been changed so only one is sent

Closes #24131

While loading, the entire dropdown is greyed out. However, The focus is on the text input, so user can input text. Move the focus event to after loading finished to fix this.

CoffeeScript generated JavaScript contains some unreadable
expressions. This refactoring addresses complex uses of
the exestential operator that produced unreadable Javascript.

- Fixes an issue where `renderItem` is called several times even when
  not required
- Increased performance when rendering dropdown items

Closes #21110

Stop the global search form's default "action" from fighting with
dropdown items when using the keyboard to navigate the dropdown.
`e.preventDefault()` is now called on the enter key when a dropdown item
is already selected.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Replace single quotes with escaped single quotes and write spec to test multiple lable selection over a page load

Added spec for removing label with space and single quote

Fixed removing label with single quote

Restore changes introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/581

Completed project filter dropdown, still need to move it from inline to ProjectSelect.js (or different)

Closes #15552

Moved `.option-hidden` to exist only in `SELECTABLE_CLASSES` as `NON_SELECTABLE_CLASSES` is used to re-show previously hidden elements