bugfix: bad tooltips and structure board areas positions

When the sideframe is opened, tooltips and structure board areas are
badly moved to the right.

As tooltips and area are "css positioned" and jquery offset function is
relative to the document (not the first relative/absolute parent), we
need to substract first relative/absolute parent offset.
Merge pull request #3649 from webu/bugfix/structureboard_areas_and_tooltips_position

bugfix: bad tooltips and structure board areas positions
fixed documentation of Django requirements for 3.0.x
Merge pull request #3652 from ojii/support/3.0.x

fixed documentation of Django requirements for 3.0.x