Make diff type customizable

Adds a switch to accounts for users to be able to select side-by-side diffs instead of inline diffs.  jsdifflib already supports this (through the `viewType` parameter); all I did was:
 1. add the flag to the model/database
 1. update methods that create/update accounts
 1. update forms that create/update accounts
 1. propagate the setting to the controllers
 1. propagate the setting to the twirl templates

My testing shows that a sign-out/sign-in is necessary after changing the `inlineDiff` setting for the results to be visible.