Remove "Misc" tab from Special:Preferences

It has served us faithfully for many years, but now, after most of its
contents have been moved to other tabs, it only contained one section
with two options related to diffs in it. Moved these two as well to
"Appearance" tab where they rightfully belong.

Also moved some code potentially adding items to the personal/i18n
section out of the misc part and to the personal part, where it should
have always been in the first place. (This actually appears to be dead
code, as $extraUserToggles doesn't seem to be defined for any language
- marked as FIXME.)

The Preferences::miscPreferences() function doesn't do anything now,
but kept it and its only call just in case for backwards-compatibility.

Bug: 52084
Change-Id: I537abc9884e18acf3ff9a425241de4607dfd7c33
Merge "Remove "Misc" tab from Special:Preferences"