Asset search and i18n fixes

[davec]

The assets search form was POSTing to edit_admin_asset_path when the Search button was pressed or the user hit Enter in the form, which wasn't the expected behavior. I changed the form to perform the same action as the observe_field action. (Note: I also made this change in the 0.8 branch, which is where I first encountered the problem.)

There's also 2 commits to the rake tasks from 2 months ago related to the i18n work that I apparently forgot to send you a pull request for.