Update tag line to "Built using Open Source, Sustained and Supported through Open Collaboration"

Back in 2012 when we closed #1859094: Replace Drupal Block with Community Media Block, we settled on the tagline "Powered by CMDrupal: Built on Open Source, Sustained by Collaboration". I really didn't give the actual text that much thought, but I was just adding some background information to https://www.drupal.org/project/cm_agenda. That along with seeing translated tagline of "Drivs av CMDrupal: byggt på öppen källkod, utvecklat genom samarbete" on new Swedish Open Channel sites made me want to clarify our tagline to differentiate "using" open source vs. collaborating to support and sustain it openly.

I've often said that getting more people to use the CMD project doesn't help nearly as much as getting someone already using it more involved. Obviously most people will want to use something before they contribute, but explaining this to organizations who rely on converting viewers into contributors should be easier than it has been.

Open conversations are critical to progress in an open source project. Private, closed conversations about building solutions USING open source are not good. Openwashed solutions that make people believe that something is open when it really isn't are even worse.

There are a number of open conversations going on right now about Backdrop that anyone who's used Drupal can provide their 2¢ about like Improving the Permissions UI (there are mockups to look at!) and Limiting the Questions Asked During the Install Process. While some of the Backdrop discussions are very technical, they've made much more of an effort to keep conversations accessible to site builders.

@westis and I went out of our way to develop the Community Media Agenda project openly. The discussions we had were almost exclusively on IRC and the issue queues and anyone could follow the progress looking at http://dev-media-event-testing.pantheon.io/. @westis and I have worked together on several projects in the past, so not everyone can operate that way... but they can certainly do more to openly discuss development plans than they are now.

While "Powered by CMDrupal: Built using Open Source, Sustained and Supported through Open Collaboration" is longer, I think the changes emphasis something that is really important.

Thoughts?