Add a maximum resolution setting for image effects

Scenario:

A user has uploaded a very large image, and wants to apply an image effect to it, such as drop-shadow

Result:

The larger the image, the more CPU is required to process the effect.  Confluence also times out the transformation after 10-12 seconds.  During this time, CPU usage spikes, and can cause stability problems for Confluence if the server is already busy.

Confluence will become unresponsive during the time when the image transformation is being applied, and this may temporarily render the instance unusable.

Suggestion:

Add a setting or JVM parameter to set a maximum resolution for image effects.  

Workaround:

Use the JVM parameter to set a max file size for image effects:

-Datlassian.image_filter.transform.max_data_size=1000000

The above code, when added to your JVM start-up parameters, will set a max size of 1MB for image transformations.  If the user attempts a transformation on a file that is larger than 1MB, the transformation will be cancelled and they will see a message that the file is too large.