Also implemented same behavior on the Issue Boards assignee block.
Fixed the default state for search/filter dropdown for issuables.

Closes #29490

Rather than constructing & then deconstructing, we know just hide & show
the sidebar. This is done so we dont' have a memory leak on the frontend
with objects getting created.

Single user autcomplete should be used only for existing users with
digital ID provided. Now js code puts any input into generating user URL
which can lead to 500 error because routing like this does not exists:

GET "/autocomplete/users/whatever@example.com.json".
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Need to get working the subscription
Styling updates

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

* Respect parameters of each individual select,
  instead of the global.
* Update skipped users in approver search when removed from default approvers.

Pass an array of user IDs in the `skip_users` param to have them
excluded from the results (unless they are explicitly included through
the `current_user` or `author_id` params).