[WIP] Add Entity Audit diff to support revision comparison

Added a `compare action` to the history view, provided by the Entity Audit implementation.

This action will generate a view like the `view revision` action, but with two columns showing the differences between two revisions. (Marked row heading).

Not really sure if I should submit these changes as a pull request, so please advise
You could remove the following lines:

```php
$currentRevision = false;
if(count($revisions) > 0){
    $currentRevision = $revisions[0];
}
```

and write it in a single line on render() parameters array:

```php
'currentRevision' => $revisions ? current($revisions) : false;
```