Allow for better memory management in Databases page

[mmcauliffe]

With all the memory related issues recently on roquefort related to running multiple large databases at once, it might be a good idea to have explicit memory management in ISCAN for each database, i.e., allow the user to specify 1-2GB for small corpora, 10GB for large corpora, etc (as well as letting the user know how much is available/currently used by other programs).