increase max length of sql tables

mysql doesn't have this limit anyways.

necessary for one plagiarism plugin.
force students to set a student id

(matrikelnummer)
mark field idnumber as required
remove useless fields from form

Conflicts:

	user/editlib.php
further simplify user profile form
restore description field as removing that breaks creating  lcal new users
remove misleading header on login page

"Bitte registrieren Sie sich" is wrong, as students
cannot register themselves.
fix: show nav bar on cource category page

important, as we don't show the navigation block.
without this fix, users won't have a link to go back to
the homepage
force download of PDF files

see https://tracker.moodle.org/browse/MDL-29987 for details