*8825* Implement plugin gallery

Implement the plugin gallery.
Still needs steenking badges.
For now, the plugin list is fetched from http://pkp.sfu.ca/ojs/plugins.xml (where a sample file currently exists).
Yes, will tinker. Looks like this is easy: http://stackoverflow.com/questions/19257426/jquery-ui-tabs-update-url-for-dynamic-tabs