Group issues search and filter bar

• Take the project issues (and merge requests) search and filter bar, and port it over to the group issues page.
• For this issue, do it only for the group issues page. Group merge requests will be a future issue.
• Filter for author, assignee, milestone, labels, and weights
• The filter scoping is exactly the same as the existing group issues list page. None of that is changed with this issue.
• For labels, we will follow https://gitlab.com/gitlab-org/gitlab-ce/issues/33575#note_35797879. That is, if the same name is for multiple labels, they will be combined in the dropdown into a multi-colored label. This is the same exact design for the existing dropdown UI. But once the user has selected that from the dropdown, the label in the search bar becomes grey.