Add Google's diff patch match js library

So that we can do inline diffs without the new lines
remove diffy and use google's js diff library

This enables editors to see changes inline more akin to MS words
'redline'. The unix diff library duplicates lines which can make it
harder to see the actual change. To achieve this we use the default
settings of dpm which are:

  Diff_EditCost = 4;
  Match_Threshold = 0.5;

And the diff_cleanupEfficiency algorithm.

The API docs are here:
https://code.google.com/p/google-diff-match-patch/
Increase edit cost to make diffs more readable
Make the insert and delete styles really like word track changes
Tidy up diff HTML / JS.
Merge pull request #894 from alphagov/tidy-google-diff-path-match

Tidy up diff HTML / JS.
Merge pull request #864 from alphagov/google-diff-path-match

Replace Diffy with google diff match patch (dmp) js library