Add OAuth.deleteUserData method

Removes the user's oAuthId-to-uid mapping from Redis upon user deletion.
This fixes an issue where OAuth services could not link to a user if a
user account was deleted and then re-created.
Fix OAuth.deleteUserData to use filter hook

The action:user.create hook in NodeBB was recently changed to
filter.user:create, which allows the OAuth.deleteUserData method to remove
OAuth data for the user more directly.
Merge branch 'master' of github.com:Fastinyoh/nodebb-plugin-sso-oauth into Fastinyoh-master

Conflicts:
	plugin.json