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 :(.