Porting the code to 2.7

        * Fix warnings related to context, settype
        * Fix header displayed before redirection
        * Fix upgrade script missing 'return true' at the end
        * editor file support + many various fix on the fly
        * fix assign user page not working
        * add file support to notes in the assign user page
        * add missing lang string for helpdesk permissions
        * Add some default values to the email plain text / title settings so administrator can reset the original value
fix grab redirect url
Downgrade required version to 2.5.5+
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