LTS (3.5) requires previous LTS (3.1)
This just deletes all the upgrade steps previous to 3.1.0. Some
small adjustments, like adding missing MOODLE_INTERNAL or tweaking
globals can also be applied when needed.
Also includes an upgrade step to prevent upgrading from any
version < 2016052300 (v3.1.0) as anti-cheating measure.
Next commit will get rid of/deprecate all the upgradelib functions
not used anymore in codebase.