Add dependency on play-previewauth 0.1.56-SNAPSHOT which includes filter
Add OAuthLoginController for preview
Add login endpoints to preview
Add public assets for login on preview
Add AuthFilterWithExemptions into preview filters
Add OAuth credentials for preview
Add preview oauth clientid and callback url
Update OAuthLoginController to use config credentials
Add admin_main.scala.html
Rename admin_main to preview_auth
Use new name
Update GuardianConfigurationTest hash
Cut down on the CSS
No longer using css class .is-authed
Add FilterExemptions for _healthcheck and old article
Remove /uk from preview healthcheck
Merge branch 'preview-oauth'

Conflicts:
	common/conf/env/DEV.properties
	common/test/common/GuardianConfigurationTest.scala