Making code compliant with GLPI v0.84.

The list of most required changes were found on GLPI Wiki (https://forge.indepnet.net/projects/plugins/wiki/Fr_Plugin083to084)
Others where found by watching PHP erors/notice while running plugin and analysing commits on GLPI project.
Fixed Html::header() usage: obsolete second parameter wasn't skipped.
Merge branch 'bugfix/1-no_submenu' into develop
Merge branch 'migration-glpi_0.84' into develop
CSRF compliance: updated forms and setup functions.
Merge branch 'evol/2-csrf_compliance' into develop