Checking application access

* Unless the user is on the root page, they will be checked for
  application access priveleges
* Updated `DapperClient` to deal with lookups on users who are not in
  the `Dapper` user database
* In the future, `check_access` will be extracted to DapperClient
* In the future, this method will redirect the user to an access request
  page in the Dapper application
* Removed useless `Roles` helper
* Added fancy flashes