workshop 2.0: overall codebase architecture cleanup and fixing regression

Due to the recent turning all workshop components into the proper subplugins, the whole architecture including DB table names and fileareas must be reviewed and regressions fixed. Together with this, the codebase architecture, names etc. will be unified
This includes:

• /grading subplugins will be /form as they define the assessment form and how the peer grade is calculated based on the assessment form data
• tables workshop_forms_ {strategyname} are now workshopform_{strategyname}

  as workshopform_ is a separate workshop plugin

• all subplugins will have their own filearea, therefore we can get rid of workshop_forms and return to the original specification. This also simplifies dimensionid (no master and local id as currently).
• "nograding" strategy now called "comments"
• "noerrors" strategy now called "numerrors"
• consistently, any subplugin's logic class is in its lib.php