Misc: Drop support for older browsers; update support comments

That includes IE<8, Opera 12.x, Firefox<29, Safari<6.0 and some hacks
for old Blackberry.

Fixes gh-1836
Fixes gh-1701
Refs gh-1815
Refs gh-1820
I don't know that we've ever established a consensus on this, but I personally see more value in `IE<9` (i.e., preserving fix information even for browsers that have rotated out of our support window).