More Grunt updates, fixes `grunt watch` task not RTL'ing or minifying CSS files.

* Use explicit CSS file names, avoids ambiguous LTR CSS (avoids LTR = `*.css` including `bbpress-rtl.css`)
* Remove `cssmin:colors` and include Mint and Evergreen CSS minification in `cssmin:core`
* Remove `cssjanus:colors` and include Mint and Evergreen RTL CSS in `cssjanus:core`
* Use `BUILD_DIR` (not `SOURCE_DIR`) to compile RTL CSS `cssjanus`
* Use `BUILD_DIR` (not `SOURCE_DIR`) to compile and LTR & RTL minified CSS with `cssmin`
* Add Grunt tasks `cssjanus:core`, `cssmin:ltr` and `cssmin:rtl` to `grunt watch` -> `watch:colors` task
* Add Grunt tasks `cssmin:ltr` and `cssmin:rtl` to `grunt watch` -> `watch:rtl` task
* Reorders `grunt build` and `grunt build-release` tasks to firstly copy `bbpress.css`, and `admin.css`, then create Mint and Evergreen CSS using `colors` , then RTL all the CSS with `cssjanus:core`, then minify all the CSS with `cssmin:ltr` and `cssmin:rtl`
* Props netweb. See #2542

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5294 9866e705-20ec-0310-96e7-cbb4277adcfb
More Grunt: Switch from `grunt-pot` to `grunt-wp-i18n`
* Remove `grunt-pot` from `package.json`
* Add `grunt-wp-i18n` to `package.json`
* Adds grunt task `grunt makebot` to `grunt build` and `grunt build-release`
* Creates `bbpress.pot` in `/build`
* Requires fresh `npm install` to update dependencies 
* Props netweb. See #2542


git-svn-id: https://bbpress.svn.wordpress.org/trunk@5295 9866e705-20ec-0310-96e7-cbb4277adcfb
Bump NPM package versions in `package.json`
* `grunt-contrib-cssmin` to v0.8.0 https://www.npmjs.org/package/grunt-contrib-cssmin
* `grunt-sass` to v~0.11.0 https://www.npmjs.org/package/grunt-sass
* `grunt-phpunit` to v0.3.3 https://www.npmjs.org/package/grunt-phpunit
* `grunt-exec` to v0.4.5 https://www.npmjs.org/package/grunt-exec
* Requires `npm install` to update packages and dependencies
* Props netweb. See #2542

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5296 9866e705-20ec-0310-96e7-cbb4277adcfb