Tidy up table of organisations

* Pluralise headings
* Move create organisation action beneath heading
* Make table more legible using borders and table heading styles
* Remove empty nav container
Tidy up heading and table styles for big lists

* Move create actions beneath heading (and warning when there is one)
* Make table more legible using borders and table heading styles —
clearer divide between columns, clearer headers, clearer table
boundaries
* Remove empty nav container
Merge pull request #1702 from alphagov/table-cleanup

Tidy up heading and table styles for big lists