Describe the bug

When Gitea OAuth is used for logging in, external user ID is not properly recorded in oauth_users.remote_user_id which causes misbehavior when logging in with different external users from Gitea server.

Steps to reproduce

Steps to reproduce the behavior:

1. Login to WriteFreely with user_a;
2. Associate user_a with Gitea user gitea_user_a;
3. Login to WriteFreely with user_b;
4. Associate user_b with Gitea user gitea_user_b;
5. Attempt to login with Gitea user gitea_user_b - you will be logged in as user_a while it is expected to be user_b.

Expected behavior

Expected behavior is logging in with gitea_user_a logs into user_a while gitea_user_b logs into user_b.

Application configuration

• Single mode or Multi-user mode? Multi-user
• Database? [mysql/sqlite] MySQL
• Open registration? [yes/no] Yes and No
• Federation enabled? [yes/no] Yes

Version or last commit: v0.13.1

To solve this issue it is required to add extra auth fields into Gitea implementation code in the same way as they are available in oauth_generic.go. Pull request will be submitted in a moment.