Add support for bordered basic tables without affecting rest of XWiki platform

Right now, the border-top of all tables is set to @table-border-color (#ddd). We want to fix the tables within user content to be fully bordered ("border-top: solid 1px black"); however, we cannot do this without affecting other tables (due the ubiquity of tables in XWiki).

It would be nice to have some way to select only the basic tables in the user content without affecting other uses (i.e. the css selector "#xwikicontent .table" won't work because it also affects LiveTables, but "#xwikicontent > .table" won't select nested tables).