Add ability to insert text in a Drive page

[slauriere]

Use case: in a Drive page, add info introducing the Drive goals and contents, with links on how to use it, metadata etc, and display this content before the Drive widget itself.

This feature could be implemented either by fixing #59 and/or by adding a field in the DriveClass displaying the page content, and allowing to edit it (might raise issue on the default FileManager page which is editable only by superadmin though).