Allow login for users without oauth2

[frafra]

It would be nice to not redirect the user to OAuth2Controller as soon as they click on the Log in link, but show the usual login page with an extra button for the OAuth2 authentication, or keep two different Log in links, one for OAuth2 users, one for CKAN users. That would allow existing users to log in.

ckanext-oauth2/ckanext/oauth2/plugin.py

Lines 102 to 104 in e68dd26

	m.connect('/user/login',
	controller='ckanext.oauth2.controller:OAuth2Controller',
	action='login')

We could have an option just to map such controller to a different URL, so that it does not override the original one. Any opinion on that?