UI for creating and managing workspace templates

There currently exists support for multiple workspace templates but there is no specific UI that helps an admin to create and manage workspace templates.

The current workaround is to directly use the WikiManager application and:
1. create a new wiki using the default workspace template as wiki template
2. set the new wiki to be a wiki template
3. add a WorkspaceManager.WorkspaceClass object in the new wiki's descriptor

The new wiki will thus show up as a workspace template in the workspace creation page.