[backend] Misc layout fixes for shipping and stock pages.

- Remove colons in stock locations form label tags for consistency.
- Center table items so it looks correct.
- Add missing "required" tag for stock location name.
- Change stock transfer "remove" button to standard round version (without tooltip and table css fx due to it do not get invoked thru Ajax).
- Fix alignment of stock locations "add" button. 
- Fix untranslated "No variants added for transfer.."
- Change table column width's to respect other locales (that might have longer words, normally German and Finnish).