Remove duplicate add mapping button

* Move pagination into the mappings.any block
* Only show another add mappings button if there are no existing
mappings, ie. the button isn’t already being shown within the table
Make all secondary header rows bold

* Fixes an inconsistency between header and footer
Merge pull request #124 from alphagov/remove-extra-add-mapping-button

Remove extra add mapping button
Merge commit '1c11b4191873e681377eb60b4cbc091df0d2b430' into HEAD