Basic implementation of all study-export options.

Fixes the bug in #400, but doesn't explain ambiguous OTU labels.
typo
Add a popup explaining all study download formats.

Fixes #400.
Revise waiver+license options for new studies.

This also includes some text cleanup. Partial fix for #369.
Fix SELECT values vs. display strings.
Display applied waiver/license; offer CC0 anytime.

Fixes #369. As requested, the UI can only apply the CC0 waiver to a
study that has no waiver or license already. This is a "ratchet", and
CC0 can't be revoked here after it's been applied. (The only way out is
to cancel or close the curation session without saving changes, or to
modify the NexSON in the docstore.
Merge pull request #401 from OpenTreeOfLife/master

Fixes for study-download options and waiver/license UI. This has been tested on devtree and devapi.