Hide wagtail admin > sidebar > search (#3810)

Since the sidebar is generated via javascript / react, the best way I
could find to remove it is to use css and then hide it.

Fixes #3808

hypha/core/wagtail_hooks.py

+from django.templatetags.static import static
+from django.utils.html import format_html
+from wagtail import hooks
+
+
+@hooks.register("insert_global_admin_css")
+def global_admin_css():
+    return format_html(
+        '<link rel="stylesheet" href="{}">', static("css/wagtail_global_admin.css")
+    )

hypha/static_src/sass/wagtail_global_admin.css

+.sidebar form[role="search"] {
+    display: none;
+}