Allow filtering of organisation list

In user research it was noted that finding organisations in the list
has become difficult, it’s grown quite long. This filter helps find
organisations quickly.

* Hide table rows that don’t match the input
* JavaScript only, the field is hidden when JS is disabled
* Prevent sideways shifting when page length changes and scrollbars
hide/show
Remove rogue </p> tag
Merge pull request #250 from alphagov/filter-orgs-list

Allow filtering of organisation list