Minimize module usage

If a Python module is only needed in a single function, rather than a global
import, just import it for that function only.
Validation bugfixes and error handling

* Fix bug in verify_hashcash to search db for the token, not the digest.
* Streamline if logic in show_post.
* Add error reporting for missing/invalid hashcash token.
* Add error reporting for missing passphrase when duress is enabled.
* Add template, js, css necessary to display errors.
Add missing return statement in duplicate name checking.
Merge pull request #26 from fungus/bugfix_1

Add missing return statement in duplicate name checking.
Merge pull request #25 from fungus/verify_hashcash

Validation bugfixes and error handling
Merge branch 'master' of https://github.com/atoponce/d-note
Consistent use of passphrase, and some typo