MDL-38603 Deprecate usepeerassessment setting in Workshop

There was a plan to make use of it initially when Workshop was rewritten
for Moodle 2.0. The idea was that the Workshop could be switched into a
simplified mode where teachers only make assessments (with all the
grading strategies available). But things evolved since then and now we
have Advanced grading methods available in the new Assignment module
that solves the use case well. So, having this option available is not
only confusing but - looking at the code - pretty useless.

For now, I am just hiding the setting from the settings form and the
code always considers it as if it was enabled. In the future, the field
can disappear from the database, too.
MDL-38603 Reorganise Workshop settings form

Example submissions settings moved into a dedicated section.
Self-assessment setting moved to the assessment section. There is no
need for the dedicated Workshop features section any more. All Workshop
feedback settings moved into a dedicated section.

Reorganised grading settings - the grading strategy put into the top
place as it is the essential setting. All other three settings are
related to calculating actual grades.

No advanced setting fields in the Workshop form. Mixing collapsible
sections with advanced fields (that are not even marked as "advanced"
any more) does not work well.

AMOS BEGIN
 CPY [feedback,mod_assign],[feedbacksettings,mod_workshop]
AMOS END