If you use GPL3 licensed software in the backend, you wouldn't have to release the entire source, as long as you don't distribute the software. Which is why AGPL was created. But something like HTML UI components can't really be used without being used over the network, so not sure if it makes any difference here. Anybody know better? Edit: I see that DanielRibeiro has an answer about mobile apps.
"The GNU Affero General Public License
The GNU Affero General Public License is based on the GNU GPL, but has an additional term to allow users who interact with the licensed software over a network to receive the source for that program. We recommend that people consider using the GNU AGPL for any software which will commonly be run over a network. The latest version is version 3."
"The GNU Affero General Public License
The GNU Affero General Public License is based on the GNU GPL, but has an additional term to allow users who interact with the licensed software over a network to receive the source for that program. We recommend that people consider using the GNU AGPL for any software which will commonly be run over a network. The latest version is version 3."
http://www.gnu.org/licenses/index_html