Update QIIME DB interface.

Checks available datasets, gives selectize.js list of datasets.
Add file size threshold as user-definable setting

In case some file is just over the limit. The important thing is to
limit the handful of outrageously large compressed files whose data
would not download, unpack, or import in a reasonable amount of time
for an interactive session
minor style change
Merge branch 'update-qiime-db-interface'