Grunt: exclude BP-Default and BP-Legacy from jshint

These JS files contains a lot of legacy JS, requiring a lot of effort
to bring up to standards. It’s better to do that incrementally as part
of template improvements, rather than just to make jshint happy. For
now, let’s exclude these from further jshint checks.

Fixes #5613, see also #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8380 cdf35c40-ae34-48e0-9cc9-0c9da1808c22