Added system property refine.max_form_content_size

To allow configuring the maximum allowed form content size in Jetty. We recieved
```
java.lang.IllegalStateException: Form too large
```
when uploading over 1MB of operation data to apply-operations. We wanted to be able to increase the hardcoded limit.

Being able to configure the allowed form content size can be useful in several cases, see https://github.com/OpenRefine/OpenRefine/issues/638 and https://github.com/OpenRefine/OpenRefine/issues/863.