Dashboard now hides "mains" for Java/MapReduce jobs

Mains are now disallowed by the api for Java and MapReduce jobs.
Given that, the dashboard now hides mains when those job types
are selected.  Also adding some help text on the job create page
to clarify what is going on.

Partial-Bug: #1272506

Change-Id: I780ff0e6bf002fb7bc626da6a35619bb4e9c2aa8