Block: Clean up handling of non-User targets

Block: Clean up handling of non-User targets

The fix applied in d67121f6d took care of the immediate issue in
T208398, but after further analysis it was not a correct fix.

* Near line 770, the method shouldn't even be called unless the target
  is TYPE_USER.
* Near line 1598, it isn't dealing with a target at all.
* Near line 1813, you're not going to get a sensible result trying to
  call `$user->getTalkPage()` for a range or auto-block ID. What you
  would really need there to handle range and auto-blocks correctly is
  to pass in the User actually making the edit.

But after some pushback in code review about passing the User into
Block::preventsEdit() to make line 1813 work, we'll instead replace the
method with Block::appliesToTitle() and put the check for user talk
pages back into User::isBlockedFrom().

Bug: T208398
Bug: T208472
Change-Id: I23d3a3a1925e97f0cabe328c1cc74e978cb4d24a