Don't display the version nor the last_updated for personas (bug 1083236)

Fixes [bug 1083236](https://bugzilla.mozilla.org/show_bug.cgi?id=1083236)

I also took the opportunity to do a bit of cleanup.

The "my submissions" theme tab doesn't display the version nor the "last
updated" date for personas (the last_updated field is never set for personas). I also removed the "sort by updated" from the sort choices:
![theme_submission](https://cloud.githubusercontent.com/assets/167767/4682324/c4391e6e-561b-11e4-9ecb-631b00269991.png)


The discovery page for the themes doesn't display the version anymore, and it
displays the created date instead of the last updated date (which is always
empty):
![persona_discovery](https://cloud.githubusercontent.com/assets/167767/4681408/b91dd0dc-5612-11e4-87e5-dd0d7fd738f9.png)