This change adds a configurable maximum attachment size for file

attachments which are connected to diffs. This works very much like the

configured maximum diff size.
Our initial default is 10MB. This should be enough for most images and

documents. The limit can be changed in the diff settings page in the

admin site.
At the moment this is enforced on the server side when attempting to

create the attachment. The limit is also included in the server

capabilities object so clients can make a decision about whether to

attempt to upload. While doing this I also added the max diff size to

the capabilities.

Ran unit tests.