Bug 19710: Move plugins templates javascript to the footer

Bug 19710: Move plugins templates javascript to the footer

This patch modifies the staff client plugins template so that
JavaScript is included in the footer instead of the header.

To test, apply the patch and test the JavaScript-driven features of
the modified template:

- The plugins link should be bold in the sidebar menu.
- Choosing "Uninstall" for an installed plugin should trigger a
  confirmation alert.

Signed-off-by: Claire Gravely <claire.gravely@bsz-bw.de>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>