Improving the maxbytes setting

Before the administration interface moving to formslib, I seem to recall that the setting for maxbytes allowed a dropdown selection of upload sizes, much like the course page. Now the setting is just a text field which requires sizes to be specified in bytes (not the most intuitive thing to do).

I can't think of a reason as to why this needs to be a text field (particularly when other parts of the interface use the dropdown approach). There also seems to be the (not particularly documented) behaviour that setting this to 0 will use the servers limit.

I've created a patch to use a dropdown box in order to set the max size, rather than needing to specify in bytes in text format.