Use layout's mixins
Use layout's mixins

Because the layouts are loaded by default, we could use it in the app
theme.

Closes #529

StartPolymer/polymer-starter-kit-plus,Negashev/negash.ru,proplanr/polymer-starter-kit,klarkc/polymer-loopback-starter-kit,abdonrd/polymer-starter-kit,StartPolymer/polymer-starter-kit,ragingwind/polymer-starter-kit,mtusk/polymer-starter-kit,StartPolymer/polymer-starter-kit,stefanoantonel/brainpower,markovuksanovic/polymer-starter-kit,stefanoantonel/brainpower,StartPolymer/polymer-starter-kit-plus,gianluka79/First,SB-Technology-Holdings-International/WateringWebClient,gick/mobile-polymer-starter-2,stefanoantonel/ecofit,LM450N/polymer-starter-kit,Negashev/negash.ru,LM450N/polymer-starter-kit,SB-Technology-Holdings-International/Water,victorgb6/id-questionary,SB-Technology-Holdings-International/Water,gick/mobile-polymer-starter-2,markovuksanovic/polymer-starter-kit,gick/reveries-project,trendystore/store,victorgb6/id-questionary,anthonybrown/polymer-starter-kit,semafloor/semafloor-test-alpha,michaetto/f16,Sykkro/penny-web,stephenzsy/charon-frontend,SB-Technology-Holdings-International/Water,andrefuller/PolymerTrailerViewer,Ex-ante-Labs/shopper.ninja,Sykkro/penny-web,Serious-Rage/poligomy,semafloor/semafloor-test-alpha,gertcuykens/polymer-starter-kit,gick/mobile-polymer-starter-2,markovuksanovic/polymer-starter-kit,plusCubed/pluscubed-site,JnJSW/dttdata,stefanoantonel/ecofit,grebett/polymer-starter-kit,felixzapata/polymer-starter-kit,RoyKoh/Project-3,SB-Technology-Holdings-International/Water,victorgb6/id-questionary,gianluka79/First,flaiker/polymer-starter-kit,gick/reveries-project,stephenzsy/charon-frontend,grebett/polymer-starter-kit,stefanoantonel/ecofit,andrefuller/PolymerTrailerViewer,abdonrd/polymer-starter-kit,proplanr/polymer-starter-kit,Negashev/negash.ru,RoyKoh/Project-3,JnJSW/dttdata,semafloor/semafloor-test-alpha,plusCubed/pluscubed-site,mtusk/polymer-starter-kit,JnJSW/dttdata,alexpaluzzi/polymer-starter-kit,flaiker/polymer-starter-kit,RoyKoh/Project-3,trendystore/store,ragingwind/polymer-starter-kit,anthonybrown/polymer-starter-kit,stephenzsy/charon-frontend,ragingwind/polymer-starter-kit,gertcuykens/polymer-starter-kit,alexpaluzzi/polymer-starter-kit,klarkc/polymer-loopback-starter-kit,SB-Technology-Holdings-International/Water,gianluka79/First,trendystore/store,SB-Technology-Holdings-International/WateringWebClient,michaetto/f16,FaellCarvalho/Geo-Location,lordoftheflies/mlm-portal,Serious-Rage/poligomy,andrefuller/PolymerTrailerViewer,SB-Technology-Holdings-International/WateringWebClient,Sykkro/penny-web,Ex-ante-Labs/shopper.ninja,FaellCarvalho/Geo-Location,FaellCarvalho/Geo-Location,felixzapata/polymer-starter-kit,Ex-ante-Labs/pivot-ahead,lordoftheflies/mlm-portal,Ex-ante-Labs/pivot-ahead,alexpaluzzi/polymer-starter-kit,lordoftheflies/mlm-portal,felixzapata/polymer-starter-kit,SB-Technology-Holdings-International/WateringWebClient,michaetto/f16,gertcuykens/polymer-starter-kit,Serious-Rage/poligomy,LM450N/polymer-starter-kit,klarkc/polymer-loopback-starter-kit,proplanr/polymer-starter-kit,gick/reveries-project,grebett/polymer-starter-kit