*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).
This uses Application::getName(). For OJS 2.x and pre-3.x, both return "ojs2" and altering OJS pre-3.x to return "ojs3" would have dire effects on the upgrade process. To clean this up fully, IMO we should have it return "ojs" -- but that will require upgrade scripts to accomplish, and above dire effects preclude it. For the moment I'm content to have "ojs2" as a bit of a throwback here.