Add find in admin bookmark

https://www.pivotaltracker.com/story/show/62236338
Put find_in_admin_bookmarklet on a separate page

also simplified the instructions. Changed the code to be a hyperlink which
can be dragged to the bookmarks bar. Removed the option for a 'open in
new window' or not, I think it's confusing.
Merge pull request #1153 from alphagov/make-bookmark

Add find in admin bookmark.

https://www.pivotaltracker.com/story/show/62236338