Gitlab Geo:  Authenticate with OAuth

This changes the way Geo (#76) nodes authenticates, to remove the requirement of session sharing between nodes, as described here: #366

# Checklist

 - [x] When a secondary Geo node is created an OAuth Application is created associated to it
 - [x] When a secondary Geo node is removed the associated OAuth application is also removed
 - [x] When a user tries to login it must be redirected to the primary node using the OAuth workflow
 - [x] The user must be redirected to the "first accessed url before authentication" (as expected)
 - [x] User should be able to logout from a secondary node
 - [x] It displays authentication errors when OAuth application is not present
 - [x] It handles authentication errors when OAuth flow fails
 - [x] Solution for what to do when/if OAuth application is removed

See merge request !237