Fix admin menu display

admin menu spans all width and centers items
since we removed horizontalNav.js from the backend we need a way to replicate the previous behavior ```display: table;``` on the parent ```<ul>``` and using ```display: table-cell;``` on children ```<li>``` ensures we always use the whole width of the parent container and we span the children ```<li>``` accordingly using the same width across the given space

see #3720 for details on why of the implementation is needed.