Check for existence of th before prepending span to trs in mobile view

projectlight.initTables unconditionally prepends the contents of th
elements to table cells in the mobile view. If the table does does have a
thead.th, the function prints "undefined" in the prepended span for every
element of the table. It would be better to check if headerTextArray is
populated before preprending the span.
Only add an overview item on the horizontal navigation if there is a non-hidden child
Style contextual links
Make sure it's an array before trying to append
Merge pull request #56 from thewilkybarkid/horizontal-navigation-hidden-children

Only add an overview item on the horizontal navigation if there is a non-hidden child
Merge pull request #58 from thewilkybarkid/class-array

Cater for modules that use strings where they should be using arrays
Merge pull request #57 from thewilkybarkid/contextual-links

Style contextual links
Merge pull request #52 from alt36/check-th-existence-on-mobile

Check for existence of th before prepending span to trs in mobile view
Space out fields