Add "Log in as user" button in Admin pane on users#show.

refs #2189
Read configuration from config.defaults.yml and config.yml.

Refuse to start if any configs are set to nil in config.default.yml
and not overridden in config/environments/*.rb or config.yml.

refs #2076
Add "rake config:check" task. refs #2076
Merge branch 'master' into 2076-check-config-at-boot
Rename config[.defaults].yml to application[.default].yml
Merge branch 'master' into 2189-log-in-as-user
Merge branch '2076-check-config-at-boot'

closes #2076
Merge branch '2189-log-in-as-user'

closes #2189