Basic group multiselect support for user token

What does this MR do and why?

Basic group multiselect support for user token

Not enabled yet but handles display and updating if multiSelect: true is set for user tokens.

Does not group params from URL yet, so after submitting the tokens they will be split again.

Broke down Group filtered assignees in multiselect (!128433 - closed) into smaller parts.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. add multiselect, for example to Author token (or use !134480 (closed))

diff --git a/app/assets/javascripts/issues/list/components/issues_list_app.vue b/app/assets/javascripts/issues/list/components/issues_list_app.vue
index 3d8ed3af816f..36ae5736e98e 100644
--- a/app/assets/javascripts/issues/list/components/issues_list_app.vue
+++ b/app/assets/javascripts/issues/list/components/issues_list_app.vue
@@ -387,6 +387,7 @@ export default {
           fetchUsers: this.fetchUsers,
           recentSuggestionsStorageKey: `${this.fullPath}-issues-recent-tokens-author`,
           preloadedUsers,
+          multiSelect: true,
         },
         {

1. Select multiple filters for author on issue list page
2. Tokens are grouped together when clicking outside

When submitting the form, the tokens are split up again. This will be fixed in next MR

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