Adding length counting to manual summaries

Also on manual section summaries. The summary_length_counting.js is copied from Whitehall.

> As a user
I don't want a really long summary on the index page 
So that I can quickly reach the table of contents

Trello ticket: https://trello.com/c/aGh1DasC/103-manuals-add-character-limit-warning-to-summaries-on-manuals-2