add script to warn folks trying to test old IE using IE's unreliable emulation modes
remove semicolons :'-(
add IE warning scripts to linter config
Document that @import-ing Bootstrap can break the icon font path

Addresses part of #13429.
Merge pull request #13435 from twbs/doc-less-import-glyphicons

Document that @import-ing Bootstrap can break the icon font path
add ie-emulation-modes-warning.js to docs & examples
Merge pull request #13422 from twbs/ie-emulation-modes-warning

Warn about IE emulation modes in docs+examples