Don't show volunteer button if not logged in

Fix my own regression -- the user object should not
be set to anything if it doesn't exist.  In the template,
guard against an empty user object.

Fixes #253

Reverts 337d4a5b740ed2ebaa03ca2c3f5e747fe453019e
Merge pull request #254 from 18F/hotfix/task-volunteer

Fixes the problem for me. Merging it in.