Prevent silent timeout failure when computing pool values on "Pool library plates" page

Observed in maptest.ucsd.edu against qiita-rc db: Usually computing pool values for a single plate works fine, but occasionally for a single plate, and reliably if more than one (96-well) plate is added, clicking the "compute pool values" button produces no result (except that the button becomes permanently grayed out, and cannot be re-enabled even by changing pooling values, which should usually trigger a re-enable). Examining the JavaScript console shows that the POST to compute_pool is timing out:


Ideally, we'd make this stop timing out :) At the very least, we should provide some sort of information to the user about why the page suddenly died on them, and how to recover. (Right now, the only way I know of to recover is to exit the page, come back in, and start over :( )