[BUG] admin oauth2 source required check (#4194)

#4059 was unfortunately incomplete: some custom_url fields are currently shown, even if they are not used by the provider. Moreover the `Use Custom URLs Instead of Default URLs` is always checked by default.

Manual testing:
- go to http://localhost:3000/admin/auths
- click on `Add authentication source`
- Choose `Authentication type`: `OAuth2`
- Choose `OAuth2 provider`: `GitLab`
- verify that the `Use Custom URLs Instead of Default URLs` option is **initially unchecked**
- enable the `Use Custom URLs Instead of Default URLs` checkbox
- verify that only the fields "Authorize", "Token" and "Profile" URLs are shown (no "Email URL", nor "Tenant").
- Switch the `OAuth2 provider` to `Azure AD v2`
- verify that the `Use Custom URLs Instead of Default URLs` option is **initially checked**
- verify that only the field "Tenant" is shown (with the default "organizations").

![image](/attachments/0e2b1508-861c-4b0e-ae6a-6eb24ce94911)

Note: this is loosely based on the upstream fix https://github.com/go-gitea/gitea/pull/31246 which I initially overlooked.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4194
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

release-notes/7.0.4/fix/4059.md

@@ -1 +0,0 @@
-Wrongfully hidden "Use Custom URLs Instead of Default URLs" checkbox on Authentication Source Administration page.

release-notes/7.0.5/fix/4059.md

@@ -0,0 +1 @@
+Authentication Source Administration page wrongfully handled the "Custom URLs Instead of Default URLs" checkbox (missing checkbox, irrelevant fields) [#4059](https://codeberg.org/forgejo/forgejo/pulls/4059) [#4194](https://codeberg.org/forgejo/forgejo/pulls/4194)