Bug 1231956: Add devtools to .eslintignore. r=pbrosset

Devtools has a lot of eslint failures right now that would block us from turning
on compile time eslint checks in bug 1229588. For now I'd like to just ignore
the directory and then you can work to clean up the warnings and re-enable
checks at whatever pace you like.

--HG--
extra : rebase_source : fa20216cecf457081fc6fc65a974a73b358d1ae2

.eslintignore

@@ -18,6 +18,7 @@ caps/**
 chrome/**
 config/**
 db/**
+devtools/**
 docshell/**
 dom/**
 editor/**