Add "use strict" to all gruntfiles across projects

[ajpiano]

Grunt 0.3.17 now requires any project that is linting the gruntfile to include a "use strict" pragma as the first line, we should add one to any project that's linting the gruntfile (or all of them, really). @ronchalant offered to take care of this :)

[rwaldron]

We literally just deleted this from our grunt.js because it's incorrect.

[ronchalant]

@rwldrn I noticed that some of the grunt.js have "use strict" incorrectly - e.g. the learn site simply had

use strict;

at the head of the file. I'm fixing them as well, and will skip core leaving it to you

[danheberden]

added this issue to any remaining sites not pending pull request - @ronchalant if you have more pull requests i'd reference those specific issues per repo