Profile makeover

This pull request started as a fix to #609 but ended up fixing #627 and #628:

Changes:

* Every user now has a profile page on /account/username (was account/profile/username)
* Profile page shows the users Gravatar
* User can now add twitter handle in settings (but not in registration)
* User can choose to make email and twitter handle public to other users
* User dashboard is now the user's profile page
* Accounts are now jinja2 templates (not i18n since that relies on pull request #624)
* Top navigation bar is now more condensed to make it shorter because of i18n