Merge: lib/popcorn: introduce AuthHandler to check user session

Merge: lib/popcorn: introduce AuthHandler to check user session

### AuthHandler allows access to session user

 Inherit this handler to access to session user from your custom handler.

 For example, you need a profile handler that checks if the user is logged
 before returning it in json format.
 ~~~nit
 import popcorn::pop_auth

 class ProfileHandler
	super AuthHandler

	redef fun get(req, res) do
		var user = check_session_user(req, res)
		if user == null then return
		res.json user.json
	end
 end
 ~~~

 By using `check_session_user`, we delegate to the `AuthHandler` the responsability
 to set the HTTP 403 error.
 We then check is the user is not null before pursuing.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #2271
Reviewed-by: Jean Privat <jean@pryen.org>