MDL-38559 Reorganise course completion settings form

This is a result of a pretty intensive effort to make the form less
sucky, given the completion strings mess around. It partially solves
MDL-39419 too as it clarifies the course completion link in the course
administration block.

I must admit and record here that I'm pretty desperate by the current
state of the core_completion and how strings from it are inconsistently
used at various places with different meanings. For example the 'Completion
tracking' may mean the mode of how activities are tracked within the
course as well as the overall feature of course and activities tracking.
While a same phrase can be used in English for both this meanings, not
all languages have such luck and translating it is a pain in the ass.

Finally, let me give the credit for wording and UI design suggestions to
Helen Foster and all others who helped with this. This has been one of
the most ugliest forms in Moodle and we believe we made it better
(although we know it's far from perfect).
MDL-38559 Fix the ability to disable the grade condition in the course completion settings form