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
Backport MOODLE_27_STABLE to MOODLE_22_STABLE

* add back previous/next
* fix the file serving script to support the 2.2 version of send_stored_file()
fix grab redirect url
Update README
Port watcher/hd_user tables + file api support + some previous 2.X improvement - it is a massive mix of many commits from Jon Doane/David Zaharee/Jerome Mouneyrac
Downgrade the required version to the first 2.2
Fix various warnings and errors related to 2.7
Remove bad character
Bump version number for last upgrade block
Remove settype warning + empty variable warning
Check Moodle version for the fullname warnings by pass
Minimum Moodle version is the first 2.5
Fix empty user id for external user when sending email + create a function to send email to external user without warnings
Fix warning when no watchers on reply
Update the README with correct branch information
Change the required version for 2.6
Merge branch 'MOODLE_27_STABLE' of https://github.com/mouneyrac/moodle-block_helpdesk into MOODLE_27_STABLE