Verify that new oauth functionality works.

[dessalines]

Requirements

• [x] This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• [x] Please check to see if this request (or a similar one) already exists.
• [x] It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

We added new createOauthProvider and authenticateWithOauth endpoints, but these need to be fully tested for 1.0

[Nutomic]

I enabled OAuth login on voyager. To use it follow these steps:

1. Open https://voyager.lemmy.ml/signup?sso_provider_id=1
2. Enter a username
3. Click on "Create Account (Github)"
4. You are redirected to Github
5. Click on "Approve"
6. You are redirected back to Lemmy and logged in, can browse and post

Out of these the first step needs changes. It should be necessary to provide this paramter manually in the URL, and from what I can see this option isnt available anywhere in the UI. So SignupProps.sso_provider_id should be removed, instead all available OAuth registration buttons should always be visible. When clicking, a username must be provided.