Grunt: overhaul tasks config.

* Removes legacy watch grunt tasks. The watch configuration was previously removed in r8550.
* Add `extDot: 'last'` option to Grunt tasks cssjanus, uglify, and cssmin. Fixes support with files with dot filenames.
* Consolidates the cssmin tasks into one.
* Task `build` now correctly verifies JS in the `src` folder. Previously, it was only validating files inside `build.`
* Fixes an issue where the RTL CSS of the file jquery.autocompletefb.css was created using the filename jquery-rtl.css.
* Don't parse bp-legacy's JS with jshint until improvements are made.

Includes files changed as a result of this Grunt change.

Fixes #5821, props netweb.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8954 cdf35c40-ae34-48e0-9cc9-0c9da1808c22