record_editor: fix javascript on /record/edit

Patch along the lines of #1901 to fix #2143.

- configures a `jquery-ui` asset containing `jquery-ui.min.js` and makes it part of the `jquery` bundle
- removes explicit `jquery-ui.min.js` loading in `legacy/bibedit/engine.py`
- reorders `header` and `_top_assets` blocks in `page_base.html` (**again?!**)

Makes the `jquery` bundle even bigger (half a megabyte now). It's just a dirty fix, not a nice solution :(.
I would suggest:
```diff
diff --git a/invenio/base/templates/legacy_page_base.html b/invenio/base/templates/legacy_page_base.html
index 50fd869..5e036e1 100644
--- a/invenio/base/templates/legacy_page_base.html
+++ b/invenio/base/templates/legacy_page_base.html
@@ -36,10 +36,10 @@
   {% endif %}
 {%- endblock title %}
 
-{%- block _top_assets %}
-  {{- super() }}
+{%- block header %}
   {%- include "base/scripts.html" %}
-{%- endblock %}
+  {{- super() }}
+{%- endblock header %}
 {%- block _bottom_assets %}
   {# The legacy page requires that the scripts are loaded from the <head>
    # rather than at the bottom of the page.
```